fix: prevent use-after-free when GC finalizes NodeQueryResult

[adsharma]

Fixes: #8

NodeQueryResult held a unique_ptr which internally owns a FactorizedTable that accesses database-managed memory (buffer pool, allocators) during destruction. If the Database was explicitly destroyed via db.closeSync() before the GC finalizer for NodeQueryResult ran, the FactorizedTable destructor would access freed memory, crashing the process.

This manifested whenever query results were not explicitly closed before closing the connection and database -- a common and reasonable usage pattern.

Fix: NodeQueryResult now holds a shared_ptr alongside its QueryResult. The database ref is acquired in AdoptQueryResult and released last in Close(), after ownedQueryResult is reset. This guarantees the Database cannot be freed until every NodeQueryResult that references it has been destroyed, regardless of GC timing.

To thread the shared_ptr through:

• NodeConnection retains its database shared_ptr after InitCppConnection() instead of dropping it immediately (the drop is moved to Close())
• Both async workers (ConnectionQueryAsyncWorker, ConnectionExecuteAsyncWorker) carry a shared_ptr and forward it to AdoptQueryResult
• QuerySync and ExecuteSync pass the connection's database ref directly
• NewInstance and GetNextQueryResultSync/Async propagate the ref to child results created when iterating multi-statement query chains

Adds regression test: discards query results without closing them, then calls conn.closeSync() and db.closeSync(), verifying the process does not crash when the GC finalizer later runs.