Add JSON wire format type conversions for UDF data types

Extend JSONEncoder.default() to handle datetime, date, timedelta, and
Decimal types that were missing from the JSON format path. These types
were added to the ROWDAT_1 binary format but the JSON encoder only
handled bytes→base64. Without this, json.dumps() raises TypeError when
a UDF returns any of these types via FORMAT JSON.

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

Author: kesmit13

singlestoredb/functions/ext/json.py

@@ -1,5 +1,7 @@
 #!/usr/bin/env python3
 import base64
+import datetime
+import decimal
 import json
 from typing import Any
 from typing import List
@@ -36,6 +38,22 @@ class JSONEncoder(json.JSONEncoder):
     def default(self, obj: Any) -> Any:
         if isinstance(obj, bytes):
             return base64.b64encode(obj).decode('utf-8')
+        if isinstance(obj, datetime.datetime):
+            return obj.strftime('%Y-%m-%d %H:%M:%S.%f')
+        if isinstance(obj, datetime.date):
+            return obj.isoformat()
+        if isinstance(obj, datetime.timedelta):
+            total_us = int(obj.total_seconds() * 1_000_000)
+            sign = '-' if total_us < 0 else ''
+            total_us = abs(total_us)
+            us = total_us % 1_000_000
+            total_secs = total_us // 1_000_000
+            ss = total_secs % 60
+            mm = (total_secs // 60) % 60
+            hh = total_secs // 3600
+            return f'{sign}{hh}:{mm:02d}:{ss:02d}.{us:06d}'
+        if isinstance(obj, decimal.Decimal):
+            return str(obj)
         return json.JSONEncoder.default(self, obj)