Refactor nested functions to instance methods in Neo4j service

[Copilot]

Addresses feedback from PR #24: nested functions _collect_statistics and _clear_sync capture self from enclosing scope, creating potential issues when moved to threads via asyncio.to_thread().

Changes

• Convert _collect_statistics() and _clear_sync() from nested functions to private instance methods
• Update call sites to use self._collect_statistics and self._clear_sync

Before:

async def get_statistics(self) -> Dict[str, Any]:
    def _collect_statistics() -> Dict[str, Any]:
        # Accesses self from closure
        return {"initialized": self._initialized, ...}
    
    return await asyncio.to_thread(_collect_statistics)

After:

def _collect_statistics(self) -> Dict[str, Any]:
    """Collect statistics from the knowledge base in a synchronous manner."""
    return {"initialized": self._initialized, ...}

async def get_statistics(self) -> Dict[str, Any]:
    return await asyncio.to_thread(self._collect_statistics)

No functional changes—methods execute identically in thread pool.

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