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

[jope-bm]

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

Generated with Claude Code

[phernandez]

I thought pydandic did this for us?

[jope-bm]

I thought pydandic did this for us?

If the source datetime is naive (no tzinfo), it is serialized without Z or timezone. Which, of course, makes sense.

So maybe the real issue is that build_context is creating datetimes without timezone? I'm not sure how we'd be getting an ISO string without tzinfo otherwise.

[phernandez]

🤷🏽 it could also be a platform thing, or some weird mcp tool thing. Whatever it takes.

[jope-bm]

@phernandez This is what Claude tells me:

Yes, the build_context tool is returning dates without timezone info.

  Issue found:
  - generated_at: Returns dates with timezone info (e.g., "2025-08-22T15:59:44.292047Z")
  - timeframe: Returns dates without timezone info (e.g., "2025-08-21T09:59:44.287982")

  Root cause:
  In src/basic_memory/services/context_service.py:157, the timeframe is set using:
  timeframe=since.isoformat() if since else None,

  The since datetime object comes from parse_timeframe() which uses the dateparser library but doesn't preserve timezone
  information, so .isoformat() produces a timestamp without the timezone suffix.

  Fix needed:
  The parse_timeframe() function should return timezone-aware datetimes, or the context service should add timezone info before
  calling .isoformat().