use model_validate (#26182)

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: crazywoola <100913391+crazywoola@users.noreply.github.com>

api/core/mcp/session/client_session.py

@@ -294,7 +294,7 @@ class ClientSession(
                     method="completion/complete",
                     params=types.CompleteRequestParams(
                         ref=ref,
-                        argument=types.CompletionArgument(**argument),
+                        argument=types.CompletionArgument.model_validate(argument),
                     ),
                 )
             ),