OpenAPI schema does not validate

Our `drf-spectacular`-generated OpenAPI schema currently does not pass `manage.py spectacular --validate` and has 8 warnings. We should fix the validation issues and fix or explicitly ignore the warnings, then add CI checks for regressions.

manage.py spectacular --file schema.yml --validate output:

```
Warning: operationId "issue_retrieve" has collisions [('/api/issue/', 'get'), ('/api/issue/{issue_id}', 'get')]. resolving with numeral suffixes.
Warning: operationId "tree_retrieve" has collisions [('/api/tree/', 'get'), ('/api/tree/{tree_name}/{git_branch}', 'get')]. resolving with numeral suffixes.
Warning: operationId "tree_boots_retrieve" has collisions [('/api/tree/{commit_hash}/boots', 'get'), ('/api/tree/{tree_name}/{git_branch}/{commit_hash}/boots', 'get')]. resolving with numeral suffixes.
Warning: operationId "tree_builds_retrieve" has collisions [('/api/tree/{commit_hash}/builds', 'get'), ('/api/tree/{tree_name}/{git_branch}/{commit_hash}/builds', 'get')]. resolving with numeral suffixes.
Warning: operationId "tree_commits_retrieve" has collisions [('/api/tree/{commit_hash}/commits', 'get'), ('/api/tree/{tree_name}/{git_branch}/{commit_hash}/commits', 'get'), ('/api/tree/{tree_name}/{git_branch}/commits', 'get')]. resolving with numeral suffixes.
Warning: operationId "tree_full_retrieve" has collisions [('/api/tree/{commit_hash}/full', 'get'), ('/api/tree/{tree_name}/{git_branch}/{commit_hash}/full', 'get')]. resolving with numeral suffixes.
Warning: operationId "tree_summary_retrieve" has collisions [('/api/tree/{commit_hash}/summary', 'get'), ('/api/tree/{tree_name}/{git_branch}/{commit_hash}/summary', 'get')]. resolving with numeral suffixes.
Warning: operationId "tree_tests_retrieve" has collisions [('/api/tree/{commit_hash}/tests', 'get'), ('/api/tree/{tree_name}/{git_branch}/{commit_hash}/tests', 'get')]. resolving with numeral suffixes.

Schema generation summary:
Warnings: 8 (8 unique)
Errors: 0 (0 unique)

SchemaValidationError: 'null' is not one of ['array', 'boolean', 'integer', 'number', 'object', 'string']

Failed validating 'enum' in schema[0]['properties']['type']:
{'type': 'string',
'enum': ['array', 'boolean', 'integer', 'number', 'object', 'string']}

On instance['type']:
'null'
```

## Related Issues

- #1992

Start by running `manage.py spectacular --file schema.yml --validate` and review the reported schema validation error and eight operationId warnings. Trace the generated schema and API routes responsible; the work is done when validation passes, warnings are fixed or explicitly ignored, and CI checks prevent regressions.

Written by the indexing model from the issue text.