Standardize UUID validation across all REST API controllers

User story

As an API consumer, when I accidentally pass a malformed identifier (e.g. a typo, a slug, or a non-UUID string) to any API endpoint, I expect a clear 400 Bad Request response telling me the parameter is invalid. Instead, most endpoints return a 500 Internal Server Error with no useful message, which makes me think the server is broken rather than that my request was wrong. This is inconsistent with the behavior on /api/workflows endpoints, which do return proper errors.

Details

UUID validation before database queries is only implemented in 2 of the 9 API controllers:

• WorkflowsController validates UUIDs across all actions via a private validate_uuid/1 function
• CredentialController validates UUIDs only in its delete action via its own duplicate private validate_uuid/1 function

The remaining 7 controllers pass user-supplied ID parameters directly into Ecto/Repo queries with no format validation:

When a malformed UUID (e.g. "notauuid") reaches Ecto, it raises either Ecto.Query.CastError or Postgrex.Error. The FallbackController has no clause to handle either exception, so Phoenix returns a generic 500 response.

Test coverage is nearly absent. Only a single test across all API controller test files (credential_controller_test.exs:842) exercises a malformed UUID input. Even WorkflowsController, which has correct production validation, has no corresponding test. All other "not found" tests use Ecto.UUID.generate() (valid UUID format, just nonexistent) — these verify 404 behavior but do not test the malformed-input path.

Implementation notes

• Shared helper: Extract a single validate_uuid/1 function into lib/lightning_web/controllers/api/helpers.ex, which already exists as a home for shared API utilities. The existing implementation pattern using Ecto.UUID.dump/1 is sound and can be reused. An alternative approach would be a Plug that validates UUID-shaped path/query params before the action is called, keeping validation logic out of individual action functions.
• Replace duplicates: Remove the two private validate_uuid/1 functions in WorkflowsController and CredentialController and replace their call sites with the shared version.
• Apply consistently: Audit every controller action that accepts a UUID parameter (path or query) and add validation at the entry point of each action. The table above can serve as a checklist.
• Error response: FallbackController already handles {:error, :bad_request} → 400, so the shared helper just needs to return that tuple on validation failure.
• Test helper: Consider a shared ExUnit helper (e.g. assert_returns_400_for_invalid_uuid(conn, method, path)) to reduce boilerplate when adding malformed-UUID tests across all controller test files. Each controller should have at least one test per action confirming that malformed UUIDs return 400 with a meaningful error body.

Release notes

Fixed a bug where passing a malformed (non-UUID) identifier to most API endpoints returned a 500 Internal Server Error instead of a 400 Bad Request. UUID parameter validation is now applied consistently across all API controllers.

User acceptance criteria

• Passing a malformed UUID as a path parameter (e.g. GET /api/jobs/notauuid) returns 400 Bad Request with a JSON error body — not 500
• Passing a malformed UUID as a query parameter (e.g. GET /api/runs?project_id=notauuid) returns 400 Bad Request with a JSON error body — not 500
• All 9 API controllers validate all UUID parameters consistently
• The two existing private validate_uuid/1 implementations are replaced by a single shared helper
• Every controller action that accepts a UUID parameter has at least one test asserting that a malformed UUID returns 400
• Existing behavior preserved: valid-format UUIDs referencing nonexistent resources still return 404
• No unhandled Ecto.Query.CastError or Postgrex.Error exceptions from UUID parameter handling in any API controller

Start with lib/lightning_web/controllers/api/helpers.ex, the existing validate_uuid/1 implementations in WorkflowsController and CredentialController, and the FallbackController handling of {:error, :bad_request}. Audit the listed API controllers and their ExUnit files, beginning with credential_controller_test.exs:842. Done means malformed path and query UUIDs return 400 with a JSON error, valid nonexistent UUIDs still return 404, and every UUID-accepting action has coverage.