google_ai: Parse thought parts in Gemini responses (#31925)

Fixes thinking Gemini models.

Closes #31902

Release Notes:

- Updated Google Gemini client to match the latest API

crates/language_models/src/provider/google.rs

@@ -620,6 +620,7 @@ impl GoogleEventMapper {
                             )));
                         }
                         Part::FunctionResponsePart(_) => {}
+                        Part::ThoughtPart(_) => {}
                     });
             }
         }