fix: include root cause in MCPClientInitializationError message (#2238)

Author: aidandaly24

src/strands/tools/mcp/mcp_client.py

@@ -220,7 +220,7 @@ def start(self) -> "MCPClient":
             logger.exception("client failed to initialize")
             # Pass None for exc_type, exc_val, exc_tb since this isn't a context manager exit
             self.stop(None, None, None)
-            raise MCPClientInitializationError("the client initialization failed") from e
+            raise MCPClientInitializationError(f"the client initialization failed: {e}") from e
         return self
 
     # ToolProvider interface methods

tests/strands/tools/mcp/test_mcp_client.py

@@ -386,7 +386,10 @@ def test_enter_with_initialization_exception(mock_transport):
     client = MCPClient(mock_transport["transport_callable"])
 
     with patch.object(client, "stop") as mock_stop:
-        with pytest.raises(MCPClientInitializationError, match="the client initialization failed"):
+        with pytest.raises(
+            MCPClientInitializationError,
+            match="the client initialization failed: Transport initialization failed",
+        ):
             client.start()
 
         # Verify stop() was called for cleanup