fix: connector close leaks for in-flight queries

[panavenue]

Change Description

Problem

When a Node.js process receives a termination signal (e.g., SIGTERM), typical graceful shutdown sequences call await pool.end() followed by connector.close(). However, as reported in #562, if there are active database queries mid-flight when shutdown begins, the TLS sockets remain alive in the event loop indefinitely, preventing the Node.js process from exiting cleanly.

There were three root causes contributing to this leak:

• Ignored Sockets: CloudSQLInstance.addSocket explicitly ignored sockets unless they connected via custom domain name routing (if (!this.instanceInfo.domainName) return;). Standard connections using an instance connection name were never tracked.
• Asynchronous Cleanup Race: Connector.close() resolved instance closure asynchronously in the promise microtask queue (instance.promise.then(inst => inst.close())). This ran too late, racing with database pool closures and failing to terminate active network handles synchronously.
• Incorrect Socket Event Listener: The connector registered a listener for the non-existent 'closed' socket event (socket.once('closed', ...)) instead of the standard Node.js 'close' event. As a result, sockets were never pruned from the tracking set even when closed normally, causing a silent reference memory leak.

Solution

This PR resolves the socket leaks and ensures clean shutdown of standard connection pools:

1. Universal Socket Tracking: Removed the domainName guard clause in CloudSQLInstance.addSocket to track all active TLS sockets regardless of the connection route.
2. Synchronous Instance Teardown: Updated Connector.close() to check if a cached instance is already resolved (entry.isResolved()) and close it synchronously. Unresolved instances fall back safely to asynchronous promise resolution.
3. Correct Socket Close Event: Updated the cleanup event listener to hook into Node's 'close' event, ensuring that normally terminated sockets are properly pruned from the internal set.
4. Safe Stream Teardown: Added a check to ensure socket.destroy exists and is a function before calling it, maintaining backward compatibility with mock socket streams used in some test environments.

Checklist

• Make sure to open an issue as a
  bug/issue
  before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea.
• Ensure the tests and linter pass
• Appropriate documentation is updated (if necessary)

Relevant issues:

• Fixes #<issue_number_goes_here>