feat(ai): add unexpectedToolCall finish reason and corresponding tests

packages/firebase_ai/firebase_ai/lib/src/api.dart

@@ -770,6 +770,9 @@ enum FinishReason {
   /// The candidate content was flagged for malformed function call reasons.
   malformedFunctionCall('MALFORMED_FUNCTION_CALL'),
 
+  /// The model produced an unexpected tool call.
+  unexpectedToolCall('UNEXPECTED_TOOL_CALL'),
+
   /// Unknown reason.
   other('OTHER');
 
@@ -790,6 +793,7 @@ enum FinishReason {
       'RECITATION' => FinishReason.recitation,
       'OTHER' => FinishReason.other,
       'MALFORMED_FUNCTION_CALL' => FinishReason.malformedFunctionCall,
+      'UNEXPECTED_TOOL_CALL' => FinishReason.unexpectedToolCall,
       _ => throw FormatException('Unhandled FinishReason format', jsonObject),
     };
   }

packages/firebase_ai/firebase_ai/test/api_test.dart

@@ -315,6 +315,7 @@ void main() {
       expect(FinishReason.recitation.toJson(), 'RECITATION');
       expect(FinishReason.malformedFunctionCall.toJson(),
           'MALFORMED_FUNCTION_CALL');
+      expect(FinishReason.unexpectedToolCall.toJson(), 'UNEXPECTED_TOOL_CALL');
       expect(FinishReason.other.toJson(), 'OTHER');
     });
 
@@ -1048,6 +1049,18 @@ void main() {
               FinishReason.malformedFunctionCall);
         });
 
+        test('parses unexpectedToolCall finishReason', () {
+          final jsonResponse = {
+            'candidates': [
+              {'finishReason': 'UNEXPECTED_TOOL_CALL'}
+            ]
+          };
+          final response =
+              VertexSerialization().parseGenerateContentResponse(jsonResponse);
+          expect(response.candidates.first.finishReason,
+              FinishReason.unexpectedToolCall);
+        });
+
         test(
             'parses groundingSupports and filters out entries without a segment',
             () {