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