fix: Ensure datetime serialization includes timezone info for RFC 3339 compliance

Add field_serializer methods to all datetime fields in memory schemas to ensure
proper timezone-aware datetime serialization. This fixes validation errors in
MCP tools like build_context where datetime strings were missing timezone
information.

- Added ensure_timezone_aware utility function
- Updated EntitySummary, RelationSummary, ObservationSummary, MemoryMetadata
- All datetime fields now serialize with timezone suffix (Z or +00:00)
- Handles both timezone-aware and naive datetimes by converting to UTC

Fixes #253

Co-authored-by: jope-bm <jope-bm@users.noreply.github.com>

test_datetime_fix.py

@@ -0,0 +1,93 @@
+#!/usr/bin/env python3
+"""Test script to verify datetime serialization fix."""
+
+import json
+from datetime import datetime, timezone
+from basic_memory.schemas.memory import EntitySummary, RelationSummary, ObservationSummary, MemoryMetadata
+
+def test_datetime_serialization():
+    """Test that datetime fields are serialized with timezone information."""
+    
+    # Test EntitySummary
+    entity = EntitySummary(
+        permalink="/test",
+        title="Test Entity",
+        file_path="/test.md",
+        created_at=datetime.now(timezone.utc)
+    )
+    
+    entity_json = entity.model_dump_json()
+    entity_dict = json.loads(entity_json)
+    
+    print("EntitySummary created_at:", entity_dict["created_at"])
+    assert entity_dict["created_at"].endswith("+00:00") or entity_dict["created_at"].endswith("Z"), \
+        f"EntitySummary created_at should have timezone: {entity_dict['created_at']}"
+    
+    # Test RelationSummary
+    relation = RelationSummary(
+        title="Test Relation",
+        file_path="/test.md",
+        permalink="/test-rel",
+        relation_type="relates_to",
+        created_at=datetime.now(timezone.utc)
+    )
+    
+    relation_json = relation.model_dump_json()
+    relation_dict = json.loads(relation_json)
+    
+    print("RelationSummary created_at:", relation_dict["created_at"])
+    assert relation_dict["created_at"].endswith("+00:00") or relation_dict["created_at"].endswith("Z"), \
+        f"RelationSummary created_at should have timezone: {relation_dict['created_at']}"
+    
+    # Test ObservationSummary
+    observation = ObservationSummary(
+        title="Test Observation",
+        file_path="/test.md",
+        permalink="/test-obs",
+        category="test",
+        content="Test content",
+        created_at=datetime.now(timezone.utc)
+    )
+    
+    observation_json = observation.model_dump_json()
+    observation_dict = json.loads(observation_json)
+    
+    print("ObservationSummary created_at:", observation_dict["created_at"])
+    assert observation_dict["created_at"].endswith("+00:00") or observation_dict["created_at"].endswith("Z"), \
+        f"ObservationSummary created_at should have timezone: {observation_dict['created_at']}"
+    
+    # Test MemoryMetadata
+    metadata = MemoryMetadata(
+        depth=1,
+        generated_at=datetime.now(timezone.utc)
+    )
+    
+    metadata_json = metadata.model_dump_json()
+    metadata_dict = json.loads(metadata_json)
+    
+    print("MemoryMetadata generated_at:", metadata_dict["generated_at"])
+    assert metadata_dict["generated_at"].endswith("+00:00") or metadata_dict["generated_at"].endswith("Z"), \
+        f"MemoryMetadata generated_at should have timezone: {metadata_dict['generated_at']}"
+    
+    print("✅ All datetime serialization tests passed!")
+    
+    # Test with naive datetime to ensure it gets converted to UTC
+    naive_dt = datetime(2025, 8, 22, 15, 30, 45, 123456)
+    entity_naive = EntitySummary(
+        permalink="/test-naive",
+        title="Test Naive Entity",
+        file_path="/test-naive.md",
+        created_at=naive_dt
+    )
+    
+    entity_naive_json = entity_naive.model_dump_json()
+    entity_naive_dict = json.loads(entity_naive_json)
+    
+    print("Naive datetime converted to:", entity_naive_dict["created_at"])
+    assert entity_naive_dict["created_at"].endswith("+00:00") or entity_naive_dict["created_at"].endswith("Z"), \
+        f"Naive datetime should be converted to UTC: {entity_naive_dict['created_at']}"
+    
+    print("✅ Naive datetime conversion test passed!")
+
+if __name__ == "__main__":
+    test_datetime_serialization()