fix(serve): give a request that fails pydantic validation an error reason

[vinismarques]

Summary

A request that fails pydantic validation is refused with 400, but its body had no error, the field a ValidationError refusal carries. A caller reading error got nothing to show.

 @app.exception_handler(pydantic.ValidationError)
-  400 {"detail": [...]}
+  400 {"error": "stems: Input should be a valid integer", "detail": [...]}

detail is unchanged. Version 5.2.2.

Evidence

• Before (c9d58fe): test_an_invalid_request_is_refused_with_a_reason_naming_the_field fails with KeyError: 'error'.
  After: it passes; full suite 93 passed, ruff and ty clean.

Merge Danger

Door: two-way

Blast Radius: small

Adds a field to one error response. A client that branches on error being absent for this case would now see it.