SubscribeToTask HTTP method mismatch between proto and spec

There is a mismatch between the .proto file and the A2A specification regarding the HTTP method for `SubscribeToTask`.

The protobuf definition uses GET:
```
Protocol Buffers
rpc SubscribeToTask(SubscribeToTaskRequest) returns (stream StreamResponse) {
option (google.api.http) = {
get: "/tasks/{id=*}:subscribe"
additional_bindings: {
get: "/{tenant}/tasks/{id=*}:subscribe"
}
};
}
```
However, the [A2A Specification](https://a2a-protocol.org/latest/specification/#113-url-patterns-and-http-methods) indicates it should be a POST request.