fix: jsonrpc client send streaming request header and timeout field (#475)

The underlying `aconnect_sse` library will overwrite the header and
timeout of what's set in the `httpx_client`.

From a user using adk `a2a_remote_agent` library, they would pass in these
information in the `httpx_client`. I have tried this fix in my local
setup.

Author: cyy12345

src/a2a/client/transports/jsonrpc.py

@@ -147,7 +147,12 @@ async def send_message_streaming(
             context,
         )
 
-        modified_kwargs.setdefault('timeout', None)
+        modified_kwargs.setdefault(
+            'timeout', self.httpx_client.timeout.as_dict().get('read', None)
+        )
+        headers = dict(self.httpx_client.headers.items())
+        headers.update(modified_kwargs.get('headers', {}))
+        modified_kwargs['headers'] = headers
 
         async with aconnect_sse(
             self.httpx_client,