fix(auth): expose cookie OAuth model for OpenAPI

[gaoflow]

Summary

• add the OAuth2 OpenAPI model expected by FastAPI's security dependency handling to OAuth2PasswordBearerWithCookie
• cover schema generation for the cookie OAuth dependency so FastAPI OpenAPI output no longer raises an AttributeError

Closes #2961

Tests

• uv run --all-packages --all-extras --dev pytest backend/tests/auth/test_cookie.py -q
• uv run --all-packages --all-extras --dev pytest backend/tests/auth backend/tests/test_callbacks.py -q
• uv run pytest --cov=chainlit from backend/
• uv run scripts/lint.py
• uv run scripts/format.py --check
• uv run scripts/type_check.py
• git diff --check

AI assistance was used under my direction.

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Summary by cubic

Expose the OAuth2 OpenAPI model in OAuth2PasswordBearerWithCookie so FastAPI generates a valid security scheme and OpenAPI no longer raises an AttributeError.

• Bug Fixes
  • Set self.model to an oauth2 password flow with tokenUrl and empty scopes, enabling FastAPI’s security dependency to emit a valid OpenAPI schema.
  • Added a test to assert the OAuth2PasswordBearerWithCookie security scheme appears in OpenAPI as expected.

^{Written for commit ef05e52. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}