verify: dispatcher non-AdcpError wrap path preserves context echo on error

[bokelley]

Context

PR #560 fixes context-echo on the AdcpError raise path in serve.py and a2a_server.py. It does NOT touch dispatch.py. Need to verify the dispatcher's exception-wrap path doesn't bypass the new echo.

The concern

In src/adcp/server/serve.py:1809-1810:

except Exception as exc:
    if DecisioningAdcpError is not None and isinstance(exc, DecisioningAdcpError):
        return build_mcp_error_result(exc, params=kwargs)  # type: ignore[return-value]
    raise

The bare raise covers the non-AdcpError case. Two paths a non-AdcpError exception can take:

1. Wrapped upstream as DecisioningAdcpError("INTERNAL_ERROR") by dispatch.py:1163 (_internal_error_details wrap). In this case the exception reaching serve.py IS DecisioningAdcpError and the new echo path runs. ✅
2. Re-raised as generic Exception through some other code path that skips the dispatch wrap (middleware-direct, executor-level, etc.). In this case the bare raise propagates and the wire eventually carries an MCP-level error without context echo. ❌

What to verify

• Walk every code path between platform.method() invocation and the serve.py:1798 except Exception to confirm non-AdcpError exceptions are always wrapped to DecisioningAdcpError("INTERNAL_ERROR") before they escape dispatch.
• If any path bypasses the wrap (test-controller, middleware short-circuit, asgi_middleware raising), file a follow-up to add the echo at that catch site too.

Acceptance

A regression test that raises ValueError("oops") from a handler with a request context field and asserts the wire response carries that context (whether wrapped as INTERNAL_ERROR or surfaced as a transport-level error).