Suppress 4XX catch-all response for explicit error codes

[moutonjeremy]

This pull request updates the OpenAPI spec generation logic to better handle how error responses are documented, particularly regarding the legacy 4XX catch-all response versus explicit error status codes. The changes clarify and enforce that when explicit error codes are provided, the generic 4XX catch-all is omitted, resulting in a more accurate and less confusing API specification. Additionally, new tests ensure this behavior is correct and maintains backward compatibility.

OpenAPI error response handling:

• The OpenAPI spec generator (fiberoapi.go) now omits the legacy 4XX catch-all response when explicit error codes are provided via OpenAPIOptions.Errors, ensuring only the enumerated status codes appear in the spec. If no explicit errors are declared and a non-empty TError is present, the 4XX catch-all is still emitted for backward compatibility.
• The documentation (README.md) is updated to explain the new behavior: the 4XX catch-all is suppressed when explicit errors are declared, and only those status codes appear in the spec.

Testing and validation:

• Two new tests are added in custom_errors_test.go:
  • One verifies that the 4XX catch-all is suppressed when explicit errors are declared.
  • The other ensures the legacy 4XX catch-all is still emitted when no explicit errors are declared, preserving backward compatibility.

[Copilot]

Pull request overview

This PR refines OpenAPI spec generation for error responses by suppressing the legacy 4XX catch-all response when a route explicitly declares per-status errors via OpenAPIOptions.Errors, while keeping the 4XX catch-all for backward compatibility when no explicit errors are provided.

Changes:

• Update spec generation to only emit the legacy 4XX catch-all when TError is non-empty and no explicit OpenAPIOptions.Errors are provided.
• Document the updated behavior in README.md.
• Add tests covering both the suppression case and the backward-compatible legacy case.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
README.md	Documents that 4XX is omitted once per-status Errors are declared.
fiberoapi.go	Suppresses the legacy 4XX response when explicit per-status errors are present.
custom_errors_test.go	Adds tests ensuring 4XX is suppressed with explicit errors and preserved without them.

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