fix: catch RecursionError in safe JSON serialization helpers

Applies the same guard to all three safe-JSON helpers (lite_llm and the
two telemetry serializers) so deeply nested input cannot escape as an
uncaught RecursionError.

Close #5411

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 933964930

Author: GWeale

src/google/adk/models/lite_llm.py

@@ -655,7 +655,7 @@ def _safe_json_serialize(obj) -> str:
   try:
     # Try direct JSON serialization first
     return json.dumps(obj, ensure_ascii=False)
-  except (TypeError, ValueError, OverflowError):
+  except (TypeError, ValueError, OverflowError, RecursionError):
     return str(obj)
 
 

src/google/adk/telemetry/_experimental_semconv.py

@@ -190,7 +190,7 @@ def _safe_json_serialize_no_whitespaces(obj: object) -> str:
         ensure_ascii=False,
         default=lambda o: '<not serializable>',
     )
-  except (TypeError, ValueError, OverflowError):
+  except (TypeError, ValueError, OverflowError, RecursionError):
     return '<not serializable>'
 
 

src/google/adk/telemetry/_serialization.py

@@ -37,7 +37,7 @@ def safe_json_serialize(obj: object) -> str:
     return json.dumps(
         obj, ensure_ascii=False, default=lambda o: "<not serializable>"
     )
-  except (TypeError, ValueError, OverflowError):
+  except (TypeError, ValueError, OverflowError, RecursionError):
     return "<not serializable>"
 
 

tests/unittests/models/test_litellm.py

@@ -709,6 +709,14 @@ def test_safe_json_serialize_circular_list_falls_back_to_str():
   assert isinstance(_safe_json_serialize(obj), str)
 
 
+def test_safe_json_serialize_recursion_error_falls_back_to_str():
+  with patch(
+      "google.adk.models.lite_llm.json.dumps",
+      side_effect=RecursionError("maximum recursion depth"),
+  ):
+    assert _safe_json_serialize({"a": 1}) == str({"a": 1})
+
+
 MULTIPLE_FUNCTION_CALLS_STREAM = [
     ModelResponseStream(
         choices=[

tests/unittests/telemetry/test_spans.py

@@ -1455,6 +1455,20 @@ def test_safe_json_serialize_no_whitespaces_circular_dict_returns_not_serializab
   assert _safe_json_serialize_no_whitespaces(obj) == '<not serializable>'
 
 
+def test_safe_json_serialize_recursion_error_returns_not_serializable():
+  with mock.patch.object(
+      json, 'dumps', side_effect=RecursionError('maximum recursion depth')
+  ):
+    assert safe_json_serialize({'a': 1}) == '<not serializable>'
+
+
+def test_safe_json_serialize_no_whitespaces_recursion_error_returns_not_serializable():
+  with mock.patch.object(
+      json, 'dumps', side_effect=RecursionError('maximum recursion depth')
+  ):
+    assert _safe_json_serialize_no_whitespaces({'a': 1}) == '<not serializable>'
+
+
 def test_use_extra_generate_content_attributes_upgraded_version(monkeypatch):
   # Arrange: Mock the presence of the new event-only context key in the contrib module
   from opentelemetry.instrumentation import google_genai