fix: capture init task ref + cancel on close for Neo4jDriver and FalkorDriver

[nandanadileep]

Summary

Both Neo4jDriver.__init__ and FalkorDriver.__init__ schedule build_indices_and_constraints() via loop.create_task() without capturing the task reference. This creates a fire-and-forget background task whose lifecycle is not managed.

Problem

1. Orphan task: The task reference is discarded, so there's no way to cancel or await it during shutdown.
2. Race condition: Callers that explicitly await driver.build_indices_and_constraints() race with the constructor-fired task, producing EquivalentSchemaRuleAlreadyExists errors.
3. Unhandled exceptions: If the task fails (e.g., connection error), "Task exception was never retrieved" warnings appear.
4. Mid-tx state: Under pytest-asyncio, when the test function returns and the event loop closes, the background task can be cancelled mid-transaction.

Fix

• Capture the task in self._init_task: asyncio.Task | None
• In close()\, cancel the task if still running and await it with suppress(asyncio.CancelledError)
• Removed redundant comments and moved the import asyncio to the top of neo4j_driver.py

Testing

• make lint passes
• make typecheck passes
• DISABLE_FALKORDB=1 DISABLE_KUZU=1 DISABLE_NEPTUNE=1 uv run pytest -m 'not integration' — 165 passed, 2 skipped (FalkorDB integration), 1 pre-existing Neo4j auth error (no local Neo4j)

Closes #1513

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[nandanadileep]

I have read the CLA Document and I hereby sign the CLA behalf on myself, e-mail: nandanadileep29@gmail.com

[Naseem77]

@nandanadileep Thanks for catching this! One gap: close() skips done() tasks, so a failed init task still leaks its exception. Calling task.exception() on done tasks too would complete it.