fix(pool): keep caller async context for queued requests

[mcollina]

Fixes #5981

Problem

Requests that wait in the Pool queue are dispatched later from whichever callback drains it (another request's connect or drain event). Client#dispatch, and the undici:request:create diagnostics channel it publishes, therefore ran in that callback's AsyncLocalStorage context instead of the caller's.

This was already the case for saturated pools (e.g. connections: 1), but since #5799 every concurrent HTTPS request is held in the Pool queue until the first connection completes ALPN negotiation, so it now affects fetch() against any https: origin by default (h1 and h2 servers alike). MSW relies on this context and broke on Node.js v26.

Fix

Queued requests are stored as a small AsyncResource subclass that captures the caller's context, and are dispatched inside it when the queue drains. Requests dispatched directly to a free client are unaffected. BalancedPool shares PoolBase, so it is covered too.

Tests

test/issue-5981.js covers HTTPS h1, HTTPS h2 (queued during ALPN) and HTTP Pool with connections: 1. All three fail without the fix and pass with it.

🤖 Generated with Claude Code

https://claude.ai/code/session_01PU6gYCHEgVgF9WeVje8SXF

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.54%. Comparing base (e960331) to head (968ce6f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5982      +/-   ##
==========================================
+ Coverage   91.53%   91.54%   +0.01%     
==========================================
  Files         113      113              
  Lines       43597    43614      +17     
==========================================
+ Hits        39908    39928      +20     
+ Misses       3689     3686       -3     

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[kettanaito]

FWIW, looks good! Thank you so much for covering it promptly.