fix: handle datetime/date in to_prompt_json

[jalaliamirreza]

Problem

Fixes #1526.

to_prompt_json (graphiti_core/prompts/prompt_helpers.py) calls json.dumps(data, ...) with no default= handler, so any datetime/date value in the serialized data raises:

TypeError: Object of type datetime is not JSON serializable

This surfaces during entity-summary regeneration —
extract_attributes_from_nodes → _extract_entity_summaries_batch → extract_summaries_batch → to_prompt_json(context['attributes']) — whenever an entity attribute holds a date (common with custom entity types that declare datetime/date fields, e.g. a contract signed_at or a person date_of_birth). The first ingest of a brand-new entity succeeds (no summary regen, so the dump never runs); the second ingest touching the same entity triggers the regen and crashes.

Repro

from datetime import datetime, timezone
from graphiti_core.prompts.prompt_helpers import to_prompt_json

to_prompt_json({"signed_at": datetime(2024, 3, 15, tzinfo=timezone.utc)})
# TypeError: Object of type datetime is not JSON serializable

Fix

Add a default= fallback to to_prompt_json:

• date / datetime → ISO-8601 (.isoformat())
• anything else json can't encode → str()

Output is unchanged for already-serializable data (the default= callable only fires for values the stock encoder rejects). Since this serializes prompt content for an LLM, ISO-8601 dates are also more readable than the raw error path.

Tests

Adds tests/test_prompt_helpers.py (pure unit, no Neo4j/LLM, runs under make test):

• datetime → ISO-8601
• date → ISO-8601
• datetime nested in lists/dicts
• plain data round-trips unchanged
• ensure_ascii=False unicode + indent preserved

make lint   # ruff check + pyright: clean
make format # clean
pytest tests/test_prompt_helpers.py  # 5 passed

Notes

• No new dependencies (stdlib datetime only).
• Bug fix to existing functionality; < 20 LOC, no RFC needed per CONTRIBUTING.