Start test returns 500 for duplicate image SHA

This start test request with an empty SHA (another image with the same SHA (empty) already must exist).
```
{
"family": "image",
"name": "cypress-noble-classic_desktop-arm64",
"version": "20260309",
"arch": "arm64",
"os": "ubuntu",
"release": "noble",
"owner": "cypress",
"execution_stage": "pending",
"image_url": "https://tel-image-cache.canonical.com/oem-share/cypress/images/noble-desktop-arm64+nvidia-latest.iso",
"sha256": "",
"test_plan": "com.canonical.certification::client-cert-desktop-24-04-automated",
"ci_link": "https://github.com/canonical/cypress-image-definitions/actions/runs/22864829317",
"environment": "202511-38084",
"needs_assignment": false
}
```
Causes `500` and this exception:
```
2026-03-10T13:37:27.980Z [test-observer-api] INFO: 10.86.58.1:51554 - "PUT /v1/test-executions/start-test HTTP/1.1" 500 Internal Server Error
2026-03-10T13:37:27.995Z [test-observer-api] ERROR: Exception in ASGI application
2026-03-10T13:37:27.995Z [test-observer-api] Traceback (most recent call last):
2026-03-10T13:37:27.995Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/pg8000/legacy.py", line 254, in execute
2026-03-10T13:37:27.995Z [test-observer-api] self._context = self._c.execute_unnamed(
2026-03-10T13:37:27.995Z [test-observer-api] ^^^^^^^^^^^^^^^^^^^^^^^^
2026-03-10T13:37:27.995Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/pg8000/core.py", line 715, in execute_unnamed
2026-03-10T13:37:27.995Z [test-observer-api] self.handle_messages(context)
2026-03-10T13:37:27.995Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/pg8000/core.py", line 835, in handle_messages
2026-03-10T13:37:27.995Z [test-observer-api] raise context.error
2026-03-10T13:37:27.995Z [test-observer-api] pg8000.exceptions.DatabaseError: {'S': 'ERROR', 'V': 'ERROR', 'C': '23505', 'M': 'duplicate key value violates unique constraint "unique_image"', 'D': 'Key (sha256)=() already exists.', 's': 'public', 't': 'artefact', 'n': 'unique_image', 'F': 'nbtinsert.c', 'L': '663', 'R': '_bt_check_unique'}
2026-03-10T13:37:27.995Z [test-observer-api]
2026-03-10T13:37:27.995Z [test-observer-api] During handling of the above exception, another exception occurred:
2026-03-10T13:37:27.995Z [test-observer-api]
2026-03-10T13:37:27.995Z [test-observer-api] Traceback (most recent call last):
2026-03-10T13:37:27.995Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 1964, in _exec_single_context
2026-03-10T13:37:27.995Z [test-observer-api] self.dialect.do_execute(
2026-03-10T13:37:27.996Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/sqlalchemy/engine/default.py", line 945, in do_execute
2026-03-10T13:37:27.996Z [test-observer-api] cursor.execute(statement, parameters)
2026-03-10T13:37:27.996Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/pg8000/legacy.py", line 281, in execute
2026-03-10T13:37:27.996Z [test-observer-api] raise cls(msg)
2026-03-10T13:37:27.996Z [test-observer-api] pg8000.dbapi.IntegrityError: {'S': 'ERROR', 'V': 'ERROR', 'C': '23505', 'M': 'duplicate key value violates unique constraint "unique_image"', 'D': 'Key (sha256)=() already exists.', 's': 'public', 't': 'artefact', 'n': 'unique_image', 'F': 'nbtinsert.c', 'L': '663', 'R': '_bt_check_unique'}
2026-03-10T13:37:27.996Z [test-observer-api]
2026-03-10T13:37:27.996Z [test-observer-api] The above exception was the direct cause of the following exception:
2026-03-10T13:37:27.996Z [test-observer-api]
2026-03-10T13:37:27.996Z [test-observer-api] Traceback (most recent call last):
2026-03-10T13:37:27.996Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/sqlalchemy/orm/session.py", line 4449, in _flush
2026-03-10T13:37:27.996Z [test-observer-api] flush_context.execute()
2026-03-10T13:37:27.996Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/sqlalchemy/orm/unitofwork.py", line 466, in execute
2026-03-10T13:37:27.996Z [test-observer-api] rec.execute(self)
2026-03-10T13:37:27.996Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/sqlalchemy/orm/unitofwork.py", line 642, in execute
2026-03-10T13:37:27.996Z [test-observer-api] util.preloaded.orm_persistence.save_obj(
2026-03-10T13:37:27.996Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/sqlalchemy/orm/persistence.py", line 93, in save_obj
2026-03-10T13:37:27.996Z [test-observer-api] _emit_insert_statements(
2026-03-10T13:37:27.996Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/sqlalchemy/orm/persistence.py", line 1233, in _emit_insert_statements
2026-03-10T13:37:27.996Z [test-observer-api] result = connection.execute(
2026-03-10T13:37:27.996Z [test-observer-api] ^^^^^^^^^^^^^^^^^^^
2026-03-10T13:37:27.996Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 1416, in execute
2026-03-10T13:37:27.996Z [test-observer-api] return meth(
2026-03-10T13:37:27.996Z [test-observer-api] ^^^^^
2026-03-10T13:37:27.996Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/sqlalchemy/sql/elements.py", line 523, in _execute_on_connection
2026-03-10T13:37:27.996Z [test-observer-api] return connection._execute_clauseelement(
2026-03-10T13:37:27.996Z [test-observer-api] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2026-03-10T13:37:27.996Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 1638, in _execute_clauseelement
2026-03-10T13:37:27.996Z [test-observer-api] ret = self._execute_context(
2026-03-10T13:37:27.996Z [test-observer-api] ^^^^^^^^^^^^^^^^^^^^^^
2026-03-10T13:37:27.996Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 1843, in _execute_context
2026-03-10T13:37:27.996Z [test-observer-api] return self._exec_single_context(
2026-03-10T13:37:27.996Z [test-observer-api] ^^^^^^^^^^^^^^^^^^^^^^^^^^
2026-03-10T13:37:27.996Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 1983, in _exec_single_context
2026-03-10T13:37:27.996Z [test-observer-api] self._handle_dbapi_exception(
2026-03-10T13:37:27.996Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 2352, in _handle_dbapi_exception
2026-03-10T13:37:27.996Z [test-observer-api] raise sqlalchemy_exception.with_traceback(exc_info[2]) from e
2026-03-10T13:37:27.996Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/sqlalchemy/engine/base.py", line 1964, in _exec_single_context
2026-03-10T13:37:27.996Z [test-observer-api] self.dialect.do_execute(
2026-03-10T13:37:27.996Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/sqlalchemy/engine/default.py", line 945, in do_execute
2026-03-10T13:37:27.996Z [test-observer-api] cursor.execute(statement, parameters)
2026-03-10T13:37:27.996Z [test-observer-api] File "/home/app/.venv/lib/python3.12/site-packages/pg8000/legacy.py", line 281, in execute
2026-03-10T13:37:27.996Z [test-observer-api] raise cls(msg)
2026-03-10T13:37:27.996Z [test-observer-api] sqlalchemy.exc.IntegrityError: (pg8000.dbapi.IntegrityError) {'S': 'ERROR', 'V': 'ERROR', 'C': '23505', 'M': 'duplicate key value violates unique constraint "unique_image"', 'D': 'Key (sha256)=() already exists.', 's': 'public', 't': 'artefact', 'n': 'unique_image', 'F': 'nbtinsert.c', 'L': '663', 'R': '_bt_check_unique'}
2026-03-10T13:37:27.996Z [test-observer-api] [SQL: /* Origin: repository.py:get_or_create */ INSERT INTO artefact (name, version, stage, family, due_date, bug_link, status, archived, comment, store, track, branch, series, repo, source, os, release, sha256, owner, image_url, assignee_id, created_at, updated_at) VALUES (%s::VARCHAR, %s::VARCHAR, %s::VARCHAR, %s, %s::DATE, %s::VARCHAR, %s, %s::BOOLEAN, %s::VARCHAR, %s::VARCHAR, %s::VARCHAR, %s::VARCHAR, %s::VARCHAR, %s::VARCHAR, %s::VARCHAR, %s::VARCHAR, %s::VARCHAR, %s::VARCHAR, %s::VARCHAR, %s::VARCHAR, %s::INTEGER, now(), now()) RETURNING artefact.id, artefact.created_at, artefact.updated_at]
2026-03-10T13:37:27.996Z [test-observer-api] [parameters: ('cypress-noble-classic_desktop-arm64', '20260309', , 'image', None, '', 'UNDECIDED', False, '', '', '', '', '', '', '', 'ubuntu', 'noble', '', 'cypress', 'https://tel-image-cache.canonical.com/oem-share/cypress/images/noble-desktop-arm64+nvidia-latest.iso', None)]
2026-03-10T13:37:27.996Z [test-observer-api] (Background on this error at: https://sqlalche.me/e/20/gkpj)
```

In addition to handling that exception better, the curious thing is that shouldn't you be able to start multiple tests for the image with the same SHA? Needs investigation.