fix: unwrap SendMessageResponse RootModel in A2A adapter

The A2A SDK's SendMessageResponse is a Pydantic RootModel that wraps
a union of JSONRPCSuccessResponse and JSONRPCErrorResponse. We need
to unwrap the .root attribute to access the actual response object
with .result or .error attributes.

This fix enables proper error handling and response processing when
communicating with A2A agents.

Verified against test-agent.adcontextprotocol.org:
- get_info() works ✓
- list_tools() works ✓
- Tool calls now properly return authentication errors instead of
  "Invalid response from A2A client"

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: bokelley

src/adcp/protocols/a2a.py

@@ -150,7 +150,11 @@ async def _call_a2a_tool(self, tool_name: str, params: dict[str, Any]) -> TaskRe
 
         try:
             # Use official A2A client
-            response = await a2a_client.send_message(request)
+            sdk_response = await a2a_client.send_message(request)
+
+            # SendMessageResponse is a RootModel union - unwrap it to get the actual response
+            # (either JSONRPCSuccessResponse or JSONRPCErrorResponse)
+            response = sdk_response.root if hasattr(sdk_response, "root") else sdk_response
 
             # Handle JSON-RPC error response
             if hasattr(response, "error"):