Added API error codes and implemented 404 on file endpoints

[wpfl-dbt]

Context

Our endpoints should provide meaningful errors. This PR adds 404, and implements it on relevant file endpoints.

On the changes to the E2E tests, the existing make test-integration wasn't working, and the existing solution would never extend to E2E tests involving the LLM, as we can't put API keys in .env.integration. A blended approach of pytest using the existing .env, but overwriting a few key vars, makes more sense.

Changes proposed in this pull request

• Adds pydantic error models inspired loosely by the RFC 9457 error specification, opening to door to following it properly in future
• Adds 404 error codes to file endpoints in core-api
• Adds unit tests to pick up these error codes
• Converts unit tests to using http.HTTPStatus instead of hard-coded error numbers
• Edits local E2E tests so they run and don't destroy your .env

Guidance to review

• Check you're happy with the RFC-lite approach
• Ensure you're happy with my specific implementation of the error codes -- have I included the right fields? Is my copy what you'd expect? Would you be happy extending these objects?
• Check you're happy with my changes to the E2E tests. Are these variables okay in pyproject.toml rather than .env.integration?

Relevant links

• RFC 9457 API error specification
• Additional Responses in OpenAPI, which shows how to add and document error codes

Things to check

• I have added any new ENV vars in all deployed environments
• I have tested any code added or changed
• I have run integration tests

[brunns]

Nitpicks.

[brunns]

You might want to check the Makefile check with @gecBurton. Aside from that, LGTM.

[gecBurton]

lovely stuff!