feat: add RFC 9457 Problem Details HTTP error handler

[miqui]

Summary

• Follows up on Support RFC9457 by default #3053 (comment: Support RFC9457 by default #3053 (comment)).
• Adds ProblemError, a struct with the RFC 9457 (Problem Details for HTTP APIs) fields: Type, Title, Status, Detail, Instance.
• Adds ProblemErrorer, an interface custom error types can implement to control how they convert into a *ProblemError.
• Adds ProblemDetailsHTTPErrorHandler(exposeError bool), a separate HTTPErrorHandler implementation (does not touch DefaultHTTPErrorHandler) that renders every error as application/problem+json, per @aldas's guidance in the discussion.
• Adds MIMEApplicationProblemJSON constant alongside the existing MIME* constants.

Design notes

• Precedence when building the response: *ProblemError (or one wrapped by err) is used as-is → else ProblemErrorer is used if implemented → else one is built from the error's HTTPStatusCoder status (default 500) and, for *HTTPError, its Message (further extended with the wrapped error's message when exposeError is true, mirroring DefaultHTTPErrorHandler's exposeError semantics). Zero-value Type/Title/Status default to "about:blank", the status text, and 500 respectively.

Test plan

• go build ./...
• go vet ./...
• go test ./...
• go test -race ./...
• New unit tests in rfc9457_test.go covering *HTTPError (with/without message, with/without wrapped errors, exposeError true/false), plain errors, *ProblemError passed directly, ProblemErrorer implementations (including nil-return safety), HEAD requests, and the committed-response short-circuit.

🤖 Generated with Claude Code

[miqui]

@aldas

usage:

e := echo.New()
e.HTTPErrorHandler = echo.ProblemDetailsHTTPErrorHandler(false) // true in dev to leak internals

e.POST("/orders", func(c *echo.Context) error {
	return &echo.ProblemError{
		Type:     "https://example.com/probs/out-of-credit",
		Title:    "You do not have enough credit.",
		Status:   http.StatusForbidden,
		Detail:   "Your current balance is 30, but that costs 50.",
		Instance: "/account/12345/msgs/abc",
	}
})

Some points to consider:

• No content negotiation — every error becomes problem+json, including for clients that asked for HTML or XML. IMHO, I think this ok.

• A wrapped *ProblemError wins over the outer *HTTPError's status, since rule 1 matches anywhere in the chain. Intentional, but surprising if you wrap a problem error in an HTTPError with a different code.
  A wrapped *ProblemError wins over the outer *HTTPError's status, since rule 1 matches anywhere in the chain. Intentional, but surprising if you wrap a problem error in an HTTPError with a different code.

	switch {
   case errors.As(err, &pe):        // target: **ProblemError
   case errors.As(err, &pder):      // target: *ProblemErrorer
   default:
   	...
   	var sc HTTPStatusCoder       // target: *HTTPStatusCoder
   	if errors.As(err, &sc) {
   		pe.Status = sc.StatusCode()
   	}
   	```

Given HTTPError{Code: 500}.Wrap(&ProblemError{Status: 403}), the chain is *HTTPError → *ProblemError:

•  Target HTTPStatusCoder (an interface): *HTTPError satisfies it, so it matches at link 1 → 500.
•  Target *ProblemError (a concrete type): *HTTPError is not that type, so it's skipped; match at link 2 → 403.

Because the *ProblemError check is rule 1 in the switch, it runs before the HTTPStatusCoder lookup ever happens. The outer error's status is never even read.
   	
   	
   	
   	
   	

[miqui]

note: no AI slop. I verified everything and made every decision.

[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.37%. Comparing base (dcb05f0) to head (a5e7ff5).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
rfc9457.go	95.83%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3062      +/-   ##
==========================================
+ Coverage   93.34%   93.37%   +0.02%     
==========================================
  Files          43       44       +1     
  Lines        4735     4783      +48     
==========================================
+ Hits         4420     4466      +46     
- Misses        192      193       +1     
- Partials      123      124       +1     

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[aldas]

LGTM