http: propagate highWaterMark to ClientRequest OutgoingMessage

[trivenay]

http.request({ highWaterMark }) passes the value to the TCP socket via createConnection() but does not set it on the OutgoingMessage's internal kHighWaterMark. OutgoingMessage._writeRaw() has two mutually exclusive write paths:

• Path A (socket connected): conn.write() — uses socket's HWM (correct)
• Path B (no socket yet): outputSize < this[kHighWaterMark] — uses OutgoingMessage's own default (64 KB) (incorrect)

Because the OutgoingMessage constructor already accepts options.highWaterMark, the fix is to set kHighWaterMark from the user's options after they are parsed in the ClientRequest constructor.

This resolves two symptoms:

1. write() returning the wrong boolean for pre-socket writes (the user's highWaterMark was silently ignored on all Node versions).
2. A deadlock on Node >= 24.16.0 where the incorrect false return sets kNeedDrain, but drain never fires because the socket was never backpressured (introduced by the stricter drain gate in http: emit 'drain' on OutgoingMessage only after buffers drain #62936).

Test results (compiled from source)

=== UNPATCHED Node v26.5.0 ===
FAIL: write() returned false (expected true)

=== PATCHED Node v27.0.0-pre (this PR) ===
PASS: write() returned true
test-http-client-highwatermark.js passes (exit 0)
test-http-server-options-highwatermark.js passes
test-http-outgoing-drain-writable-length.js passes
test-http-outgoing-end-cork.js passes
test-http-highwatermark.js passes
test-http-response-drain-cork.js passes

Minimal reproduction

node -e "require('http').createServer((req,res)=>{req.resume();req.on('end',()=>res.end('ok'))}).listen(9001)"

const http = require('http');
const req = http.request({ hostname: '127.0.0.1', port: 9001, method: 'POST', highWaterMark: 100_000 });
const ok = req.write(Buffer.alloc(64 * 1024));
console.log(`write() returned ${ok}, expected true (64KB < 100KB highWaterMark)`);
if (!ok) {
  setTimeout(() => { console.error('DEADLOCK: drain never fired'); process.exit(1); }, 3000);
  req.on('drain', () => { console.log('drain fired'); req.end(); });
} else {
  req.end();
  req.on('response', (res) => { res.resume(); res.on('end', () => process.exit(0)); });
}

Node version	write() returns	Drain fires?	Outcome
v22.x	false (wrong)	Yes (old eager drain)	HWM ignored but no hang
v26.5.0	false (wrong)	Never	DEADLOCK
This PR	true (correct)	N/A (no backpressure)	Works correctly

Fixes: #64645
Refs: #62936

[nodejs-github-bot]

Review requested:

• @nodejs/http
• @nodejs/net

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.13%. Comparing base (8c1d128) to head (a008632).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #64653   +/-   ##
=======================================
  Coverage   90.12%   90.13%           
=======================================
  Files         741      741           
  Lines      242091   242142   +51     
  Branches    45563    45563           
=======================================
+ Hits       218180   218250   +70     
+ Misses      15430    15394   -36     
- Partials     8481     8498   +17     

Files with missing lines	Coverage Δ
lib/_http_client.js	97.62% <100.00%> (+0.01%)	⬆️

... and 36 files with indirect coverage changes

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[trivikr]

LGTM

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/74985/