fix: skip drain dispatch for clientTtl-evicted clients in PoolBase

[vwong]

Problem

When a Pool is configured with clientTtl, evicted clients can still emit drain events after being closed — typically when an in-flight request completes after the client has been removed from the pool. kOnDrain has no guard against this and will attempt to dispatch queued pool items to the closed client, throwing ClientClosedError.

The race window is narrow in low-latency environments (same-region) but widens significantly with cross-region TLS connection times (hundreds of milliseconds), making the error reliably reproducible under load.

Sequence that triggers the bug:

1. Pool has connections: 1. Client C1 is active, its clientTtl expires.
2. A new request arrives → kGetDispatcher evicts C1 (client.close(), kClosed = true), creates C2.
3. C2 starts a TLS handshake. While it's connecting, further requests fill the pool queue (kQueue), because C2 has kNeedDrain = true and the connections limit is reached.
4. C1 finishes an in-flight response → resume() → emitDrain() → C1.emit('drain').
5. kOnDrain(C1) dequeues an item from kQueue and calls C1.dispatch() → ClientClosedError.

Fix

Return early in kOnDrain when the client is already closed or destroyed. The queued items remain in kQueue and are correctly drained when C2 connects and emits its own drain event.

Why connections: 2 doesn't prevent this alone

The drain listener is added in kAddClient and bound to the specific client instance. kRemoveClient removes the client from kClients but never removes the listener. So even with a second connection available, the stale listener on C1 still fires and attempts to dispatch to C1.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.45%. Comparing base (dd85997) to head (0c26a12).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5677      +/-   ##
==========================================
+ Coverage   93.43%   93.45%   +0.02%     
==========================================
  Files         110      110              
  Lines       38733    38780      +47     
==========================================
+ Hits        36190    36242      +52     
+ Misses       2543     2538       -5     

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