Handle NCBI taking too long to respond

I think the below error occurred when the answer of NCBI took too long to respond. This type of error should be handled so it does not return internal server error:

Traceback (most recent call last):
  File "/home/backend/venv/lib/python3.11/site-packages/uvicorn/protocols/http/h11_impl.py", line 403, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/backend/venv/lib/python3.11/site-packages/uvicorn/middleware/proxy_headers.py", line 60, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/backend/venv/lib/python3.11/site-packages/fastapi/applications.py", line 1054, in __call__
    await super().__call__(scope, receive, send)
  File "/home/backend/venv/lib/python3.11/site-packages/starlette/applications.py", line 113, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/home/backend/venv/lib/python3.11/site-packages/starlette/middleware/errors.py", line 187, in __call__
    raise exc
  File "/home/backend/venv/lib/python3.11/site-packages/starlette/middleware/errors.py", line 165, in __call__
    await self.app(scope, receive, _send)
  File "/home/backend/venv/lib/python3.11/site-packages/starlette/middleware/cors.py", line 93, in __call__
    await self.simple_response(scope, receive, send, request_headers=headers)
  File "/home/backend/venv/lib/python3.11/site-packages/starlette/middleware/cors.py", line 144, in simple_response
    await self.app(scope, receive, send)
  File "/home/backend/venv/lib/python3.11/site-packages/starlette/middleware/exceptions.py", line 62, in __call__
    await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)
  File "/home/backend/venv/lib/python3.11/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
    raise exc
  File "/home/backend/venv/lib/python3.11/site-packages/starlette/_exception_handler.py", line 42, in wrapped_app
    await app(scope, receive, sender)
  File "/home/backend/venv/lib/python3.11/site-packages/starlette/routing.py", line 715, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/home/backend/venv/lib/python3.11/site-packages/starlette/routing.py", line 735, in app
    await route.handle(scope, receive, send)
  File "/home/backend/venv/lib/python3.11/site-packages/starlette/routing.py", line 288, in handle
    await self.app(scope, receive, send)
  File "/home/backend/venv/lib/python3.11/site-packages/starlette/routing.py", line 76, in app
    await wrap_app_handling_exceptions(app, request)(scope, receive, send)
  File "/home/backend/venv/lib/python3.11/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
    raise exc
  File "/home/backend/venv/lib/python3.11/site-packages/starlette/_exception_handler.py", line 42, in wrapped_app
    await app(scope, receive, sender)
  File "/home/backend/venv/lib/python3.11/site-packages/starlette/routing.py", line 73, in app
    response = await f(request)
               ^^^^^^^^^^^^^^^^
  File "/home/backend/venv/lib/python3.11/site-packages/fastapi/routing.py", line 301, in app
    raw_response = await run_endpoint_function(
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/backend/venv/lib/python3.11/site-packages/fastapi/routing.py", line 212, in run_endpoint_function
    return await dependant.call(**values)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/backend/src/opencloning/endpoints/external_import.py", line 400, in genome_coordinates
    _, _, seq = await asyncio.gather(*tasks)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/backend/src/opencloning/endpoints/external_import.py", line 384, in validate_assembly_task
    sequence_accessions = await ncbi_requests.get_sequence_accessions_from_assembly_accession(
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/backend/src/opencloning/ncbi_requests.py", line 32, in get_sequence_accessions_from_assembly_accession
    resp = await async_get(url, headers=headers)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/backend/src/opencloning/ncbi_requests.py", line 13, in async_get
    return await client.get(url, headers=headers, params=params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/backend/venv/lib/python3.11/site-packages/httpx/_client.py", line 1757, in get
    return await self.request(
           ^^^^^^^^^^^^^^^^^^^
  File "/home/backend/venv/lib/python3.11/site-packages/httpx/_client.py", line 1530, in request
    return await self.send(request, auth=auth, follow_redirects=follow_redirects)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/backend/venv/lib/python3.11/site-packages/httpx/_client.py", line 1617, in send
    response = await self._send_handling_auth(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/backend/venv/lib/python3.11/site-packages/httpx/_client.py", line 1645, in _send_handling_auth
    response = await self._send_handling_redirects(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/backend/venv/lib/python3.11/site-packages/httpx/_client.py", line 1682, in _send_handling_redirects
    response = await self._send_single_request(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/backend/venv/lib/python3.11/site-packages/httpx/_client.py", line 1719, in _send_single_request
    response = await transport.handle_async_request(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/backend/venv/lib/python3.11/site-packages/httpx/_transports/default.py", line 365, in handle_async_request
    with map_httpcore_exceptions():
  File "/usr/local/lib/python3.11/contextlib.py", line 158, in __exit__
    self.gen.throw(typ, value, traceback)
  File "/home/backend/venv/lib/python3.11/site-packages/httpx/_transports/default.py", line 83, in map_httpcore_exceptions
    raise mapped_exc(message) from exc
httpx.ReadTimeout

Start with src/opencloning/ncbi_requests.py, especially async_get, and trace its callers in src/opencloning/endpoints/external_import.py, including genome_coordinates and validate_assembly_task. Reproduce or test the httpx.ReadTimeout path and define the resulting non-internal-server-error response; done when an NCBI timeout is handled without the traceback shown.