[Bug]: CancelTaskRequest.metadata never reaches AgentExecutor.cancel #1415

What happened?

CancelTaskRequest has a metadata field (a2a_pb2.pyi; the REST mapping sends the whole request as the body, post: "/tasks/{id=*}:cancel", body: "*" in a2a.proto), but nothing on the server hands it to AgentExecutor.cancel:

• RestDispatcher.on_cancel_task (src/a2a/server/routes/rest_dispatcher.py) builds CancelTaskRequest(id=task_id) and does not read the body.
• On every binding, on_cancel_task in DefaultRequestHandlerV2 reads only params.id, and the RequestContext given to cancel has no request (request=None in default_request_handler.py, no request in ActiveTask.cancel). So context.metadata is {} for a cancel, while for SendMessage it returns the request's metadata.

A caller's {"metadata": {...}} therefore never reaches the executor, whichever transport it uses. The Go SDK (gRPC) and the .NET SDK (gRPC and JSON-RPC) do pass it to the executor; only their REST bindings drop it.

Expected: context.metadata of a cancel holds the request's metadata. For REST, decode the body into CancelTaskRequest and keep the id from the path.

Read from the source at 11f1116 (v1.3.0 + 12 commits); I did not run it.

Relevant log output

n/a (found by reading the source)

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