[Bug]: a2a.proto and specification divergence on subscribeToTask

### What happened?

According to a2a.proto: subsribeToTask is a GET
`/ Subscribes to task updates for tasks not in a terminal state.
  // Returns `UnsupportedOperationError` if the task is already in a terminal state (completed, failed, canceled, rejected).
  rpc SubscribeToTask(SubscribeToTaskRequest) returns (stream StreamResponse) {
    option (google.api.http) = {
      get: "/tasks/{id=*}:subscribe"
      additional_bindings: {
        get: "/{tenant}/tasks/{id=*}:subscribe"
      }
    };
  }`
but in the specifications (https://a2a-protocol.org/latest/specification/#1132-task-operations) it is a POST.
We should clarify this

### Relevant log output

```shell

```

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