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>
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