perf: replace json.dumps serialization test with isinstance fast path (#96)

Closes #96

Replaces the per-field `json.dumps(value)` probe — which fully serialized
and discarded the result just to check serializability — with a type-check
fast path using `isinstance()`. The `_SERIALIZABLE_TYPES` tuple is defined
at module level so it's not recreated on every log call.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

utils/logging.py

@@ -24,6 +24,8 @@ JSONValue = Union[
     str, int, float, bool, None, dict[str, Any], list[Any]  # nested object  # arrays
 ]
 
+_SERIALIZABLE_TYPES = (str, int, float, bool, type(None))
+
 
 class JSONFormatter(logging.Formatter):
     """Custom JSON formatter for structured file logging."""
@@ -93,11 +95,11 @@ class JSONFormatter(logging.Formatter):
         extra_data = {}
         for key, value in record.__dict__.items():
             if key not in excluded_keys:
-                # Ensure JSON serializable
-                try:
-                    json.dumps(value)
+                if isinstance(value, _SERIALIZABLE_TYPES) or isinstance(
+                    value, (list, dict)
+                ):
                     extra_data[key] = value
-                except (TypeError, ValueError):
+                else:
                     extra_data[key] = str(value)
 
         if extra_data: