Track scheduled driver index build tasks

[RitwijParmar]

Summary

Fixes #1513 by keeping references to the constructor-scheduled index build tasks in the Neo4j and FalkorDB drivers, then cancelling/awaiting them during driver close before closing the underlying database client.

This prevents the fire-and-forget build_indices_and_constraints() task from being abandoned when a function-scoped event loop or short-lived application shuts down.

Changes

• Store the scheduled index build task on Neo4jDriver and FalkorDriver
• On close(), retrieve completed task exceptions or cancel and await pending tasks
• Add regression coverage for pending index task cleanup in both drivers

Tests

• /tmp/graphiti/.venv/bin/python -m ruff check graphiti_core/driver/neo4j_driver.py graphiti_core/driver/falkordb_driver.py tests/driver/test_neo4j_driver.py tests/driver/test_falkordb_driver.py
• /tmp/graphiti/.venv/bin/python -m pytest tests/driver/test_neo4j_driver.py tests/driver/test_falkordb_driver.py

Result: 25 passed, 1 skipped.

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[Naseem77]

@RitwijParmar falkordb_driver.py changed on main so this no longer applies. Could you rebase? Also worth syncing with #1636 which touches the same lifecycle.

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[RitwijParmar]

Rebased onto current main and pushed. The driver index task ownership and close-time cancellation are unchanged after the rebase.

I checked #1636 as well. It covers the same driver lifecycle area but a separate init-task issue, so this PR stays focused on retaining and cancelling the scheduled index-build task. Focused tests pass 27 with 1 skipped and Ruff is clean.

[RitwijParmar]

I have read and agree to the Zep CLA on behalf of myself, e-mail: ritwij.aryan.parmar@gmail.com

[RitwijParmar]

I have read the CLA Document and I hereby sign the CLA behalf on myself, e-mail: ritwij.aryan.parmar@gmail.com