[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 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)** ## Literal error text ``` application/a2a+json error bodies ignored ``` ## Environment GitHub issue a2aproject/a2a-go #447; state=open ## Trigger / context Public GitHub report in a2aproject/a2a-go (FRESH_7D). ## Source evidence - https://github.com/a2aproject/a2a-go/issues/447 (2026-09-30T11:58:10Z) — evidence_class: PRIMARY_ISSUE_REPORT — 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. Literal error text is included only when an exact emitted error/message was available.