fix: Handle EquivalentSchemaRuleAlreadyExists errors in Neo4j driver

[supmo668]

Summary

Fixes #1079

Neo4j 5.26+ throws EquivalentSchemaRuleAlreadyExists errors when creating indices in parallel, even with IF NOT EXISTS clause. This causes Docker Compose deployments to fail on startup.

Changes

• Added error handling in Neo4jDriver.execute_query() to catch neo4j.exceptions.ClientError
• Specifically checks for EquivalentSchemaRuleAlreadyExists error code
• Logs the occurrence as info instead of error
• Returns empty result to indicate success (index/constraint already exists)

Technical Details

The race condition occurs when multiple CREATE INDEX IF NOT EXISTS queries run concurrently via semaphore_gather in build_indices_and_constraints(). Neo4j 5.26+ returns an error even though the index creation succeeds.

This solution follows the same pattern already implemented in the FalkorDB driver for handling "already indexed" errors (see FalkorDriver.execute_query() lines 177-180).

Test Plan

• Code passes make lint (ruff + pyright)
• Follows existing error handling pattern from FalkorDB driver
• Integration tests require Neo4j instance (will be validated by CI)

Checklist

• Code follows project style guidelines
• Changes are focused on fixing the specific issue
• Error handling is consistent with existing patterns
• Appropriate logging added

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