fix(h1): stop unref'ing the idle socket validation immediate

[marko1olo]

This relates to...

Fixes #5600.

Rationale

Reusing an idle keep-alive socket stalls for up to a fast-timers tick before the request reaches the wire. With a pool of one connection against a 10 ms server, six sequential requests take over a second instead of the ~60 ms the server accounts for.

scheduleIdleSocketValidation moved from an unref'd setTimeout(..., 0) to setImmediate in #5499, and the unref?.() came along with it. An unref'd immediate does not hold the loop open, so on an otherwise idle loop the check phase is reached only once the next fast-timers tick wakes things up, and the queued request waits for that.

By the time the validation is scheduled, the request it exists for is already queued, so there is nothing to keep alive artificially. The immediate is a one-shot that runs on the very next check phase, and clearIdleSocketValidation cancels it when the socket goes away, so leaving it ref'd cannot delay process exit by more than that one turn — unlike the setTimeout this was inherited from.

Measured on Windows with Node 24, six sequential requests through a single-connection pool:

before   1095 ms, 1565 ms, 2045 ms   (per-request: 34, 41, 447, 66, 438, 50)
after     122 ms,  137 ms,  219 ms   (per-request: 38, 23, 15, 15, 16, 15)
server-side floor: 60 ms

Changes

• lib/dispatcher/client-h1.js: drop the unref?.() on the idle socket validation immediate.
• test/issue-5600.js: the reproduction from the issue, asserting the slowest of six sequential requests on a single-connection pool stays well under a fast-timers tick. It fails on current main with a slowest request around 425 ms.

npx borp --timeout 180000 --expose-gc -p "test/*.js" is 1439 passing here. The one failure in that run is test/http2-dispatcher.js "Should send http2 PING frames", which fails about two runs in three on unpatched main on this machine as well — unrelated to HTTP/1, and I have a separate branch for it if that is wanted.

Features

• Implemented a new feature

Bug Fixes

• Fixed a bug

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.49%. Comparing base (8453a77) to head (7fd4337).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5609   +/-   ##
=======================================
  Coverage   93.49%   93.49%           
=======================================
  Files         110      110           
  Lines       38281    38302   +21     
=======================================
+ Hits        35789    35811   +22     
+ Misses       2492     2491    -1     

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