[a2a-go] REST client ignores error bodies sent as application/a2a+json and returns generic server error

## Observed symptom [Bug]: REST client ignores error bodies sent as application/a2a+json and returns a generic server error **What happened** The REST client never decodes an A2A error body that the server sends as `application/a2a+json`. Every typed error (`TaskNotFound`, `TaskNotCancelable`, ...) surfaces as the generic `a2a.ErrServerError` ("server error"); the message and `ErrorInfo` details are lost, so `errors.Is(err, a2a.ErrTaskNotFound)` is false. **Root cause (v2.6.0, identical on main @ ebf17c5)** `internal/rest/rest.go` `FromRESTError` returns `a2a.ErrServerError` unless `Content-Type` starts with `application/json`: ```go contentType := resp.Header.Get("Content-Type") if !strings.HasPrefix(contentType, "application/json") { return a2a.ErrServerError } ``` `application/a2a+json` does not have that prefix. Success responses are not affected (the body is decoded without looking at the type), a ## Literal error text ``` application/a2a+json error bodies ignored ``` ## Environment GitHub issue a2aproject/a2a-go #447; state=open ## Trigger / context **What happened** The REST client never decodes an A2A error body that the server sends as `application/a2a+json`. Every typed error (`TaskNotFound`, `TaskNotCancelable`, ...) surfaces as the generic `a2a.ErrServerError` ("server error"); the message and `ErrorInfo` details are lost, so `errors.Is(err, a2a.ErrTaskNotFound)` is false. **Root cause (v2.6.0, identical on main @ ebf17c5)** `internal/rest/rest.go` `FromRESTError` returns `a2a.ErrServerError` unless `Content-Type` starts with `application/json`: ```go contentType := resp.Header.Get("Content-Type") if !strings.HasPrefix(contentType, "application/json") { return a2a.ErrServerError } ``` `application/a2a+json` does not have that prefix. Success responses are not affected (the body is decoded without looking at the type), and `a2asrv` accepts `application/a2a+json` requests, so only the client's error path is wrong. **Spec** 11.1 (HTTP+JSON/REST): "Content-Type: application/a2a+json SHOULD be used for requests and responses". The spec's own error example in 11.6 (404 `TASK_NOT_FOUND`) is served as `Content-Type: application/a2a+json`. It is a SHOULD, so `application/json` servers (like a2asrv) stay valid; the client just needs to accept both. The media type is registered in 14.1.1. **Reproduction** Only the `Content-Type` header changes; the server is a bare `httptest` handler returning the 11.6 body (`{"error":{"code":404,"status":"NOT_FOUND","message":"...","details":[{"@type":"type.googleapis.com/google ## Source evidence - https://github.com/a2aproject/a2a-go/issues/447 (2026-09-30T11:58:10Z) — GitHub issue #447 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:43:03.388295Z - scout_id: C81eb356b10 ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome.