fix(telemetry): serialize Pydantic BaseModel in _safe_json_serialize

_safe_json_serialize replaces Pydantic BaseModel instances with
'<not serializable>' because json.dumps cannot handle them natively.
Any tool that returns a Pydantic model has its traced output lost.

Replace the generic lambda default with a _default function that
calls model_dump(mode='json') for BaseModel instances before falling
back to '<not serializable>' for truly non-serializable objects.
BaseModel is already imported in tracing.py.

Fixes #4629

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: giulio-leone

src/google/adk/telemetry/tracing.py

@@ -116,11 +116,14 @@ def _safe_json_serialize(obj) -> str:
     The JSON-serialized object string or <non-serializable> if the object cannot be serialized.
   """
 
+  def _default(o: Any) -> Any:
+    if isinstance(o, BaseModel):
+      return o.model_dump(mode='json')
+    return '<not serializable>'
+
   try:
     # Try direct JSON serialization first
-    return json.dumps(
-        obj, ensure_ascii=False, default=lambda o: '<not serializable>'
-    )
+    return json.dumps(obj, ensure_ascii=False, default=_default)
   except (TypeError, OverflowError):
     return '<not serializable>'