fix(binder): serialize BindingError to structured JSON (#2771)

[vishr]

Problem (#2771)

A binding error returned from a handler is serialized as {"message":"Bad Request"} — the field name and the binder message are both lost.

Root cause

BindingError embeds *HTTPError but does not implement json.Marshaler. In DefaultHTTPErrorHandler the type switch runs on the HTTPStatusCoder extracted via errors.As, whose dynamic type is *BindingError. Go's case *HTTPError matches only the exact type, so a *BindingError falls through to the default branch → {"message": http.StatusText(code)}. Regression from fbfe216 (#2456).

Verified on current master:

GET /doc?docNum=abc  →  400  {"message":"Bad Request"}

Fix

Implement MarshalJSON on *BindingError so it takes the handler's existing case json.Marshaler branch (which is checked before *HTTPError). Restores the v4.10.2 structured response:

GET /doc?docNum=abc  →  400  {"field":"docNum","message":"failed to bind field value to int"}

This is the approach the maintainer outlined in the issue thread ("we could make echo.BindingError … implement json.Marshaler").

Test

TestBindingError_serializesToStructuredJSON (written first; fails on master with field=<nil>, message="Bad Request"). gofmt/vet clean; full root-package suite passes.

Fixes #2771.

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