fix: stop destroyed pool clients from dialing again

[guiguan]

The channel pool's destroy closes each client and then calls client.getChannel().getConnectivityState(true). On a closed channel, grpc-js (1.14.4) still acts on tryToConnect: it restarts name resolution and dials a new connection. Nothing references that channel any more, so the connection is never closed.

Each pooled client that is destroyed therefore leaves one connection behind. That happens on closeConnection() and when a topology change replaces the pool, which holds at least 2 clients by default.

• With TLS: every client has its own credentials, so every stray connection is a separate socket.
• Without TLS: grpc-js shares a single socket across these clients, which hides the leak.

We found this through Attu v3.0.1, which ships this SDK (3.0.6). Against a TLS Milvus, with a browser tab left open, the proxies held ~590 established connections from the one Attu pod, growing by about one a minute.

Fix

destroy only closes the client. generic-pool ignores the value destroy resolves to, so nothing depends on the removed call. It dates from the original closeConnection(), which returned the post-close state.

Test

test/utils/ChannelPool.spec.ts runs a local HTTP/2 TLS server with the repo's test certificates and needs no Milvus. It does the following, then expects 0 sessions on the server:

1. creates the real channel pool with tls.skipCertCheck;
2. connects one client;
3. releases it and drains the pool.

• Before the fix: 1 session remains.
• After the fix: 0.

test/utils Grpc, Connection and Data specs still pass.

Standalone reproduction with grpc-js 1.14.4 directly, 5 create/close cycles each:

setup	server sessions left open
close() only, TLS or plaintext	0
close() + getConnectivityState(true), plaintext	1
close() + getConnectivityState(true), TLS	5

[sre-ci-robot]

Welcome @guiguan! It looks like this is your first PR to milvus-io/milvus-sdk-node 🎉

[sre-ci-robot]

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This pull-request has been approved by: guiguan, shanghaikid

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