fix(pool): reject queued requests on end

[gilboom]

What changed

Reject connection requests that are already queued in BasePool._connectionQueue when pool.end() closes the pool.

Why

When a promise pool is saturated and additional pool.query() calls are waiting for a connection, calling pool.end() marks the pool as closed and closes the active connections, but the queued callbacks are never invoked. The promise wrappers for those queued queries can stay pending forever.

This makes queued requests behave consistently with new getConnection() calls after pool closure, which already receive Pool is closed..

Validation

• Added an integration regression test that saturates a promise pool, waits until two queries are queued, calls pool.end(), and asserts the queued queries reject with Pool is closed. instead of timing out.
• Verified the new test fails before the fix with timed out waiting for queued queries to settle; queue length=2.
• CI=1 MYSQL_PASSWORD=root MYSQL_DATABASE=mysql node --import tsx test/integration/test-pool-end.test.mts
• npm run lint

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.00%. Comparing base (f66f87e) to head (4a5e703).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4291   +/-   ##
=======================================
  Coverage   91.00%   91.00%           
=======================================
  Files          90       90           
  Lines       14521    14525    +4     
  Branches     1868     1871    +3     
=======================================
+ Hits        13215    13219    +4     
  Misses       1306     1306           

Flag	Coverage Δ
compression-0	90.27% <100.00%> (+<0.01%)	⬆️
compression-1	90.98% <100.00%> (+<0.01%)	⬆️
static-parser-0	88.73% <100.00%> (+<0.01%)	⬆️
static-parser-1	89.48% <100.00%> (+<0.01%)	⬆️
tls-0	90.45% <100.00%> (+<0.01%)	⬆️
tls-1	90.78% <100.00%> (+<0.01%)	⬆️

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