fix(ci): bump coverage heap budget + Windows npm timeout in runNpm helper (#3704)

Set NODE_OPTIONS=--max-old-space-size=6144 on the Unit coverage step so the
c8 report phase has 6 GB instead of Node's default ~4 GB heap; the Linux
Node 24 runner has 7 GB available so this leaves 1 GB headroom. Raise the
runNpm default timeout from 55 000 ms to 180 000 ms so cold-cache Windows
npm install -g runs (which take 60-90 s on NTFS + Defender) complete before
the child process is killed.

Refs #3703

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

tests/helpers.cjs

@@ -250,7 +250,7 @@ function runNpm(args, options = {}) {
   const defaults = {
     encoding: 'utf-8',
     shell: isWindows,
-    timeout: 55000,
+    timeout: 180000,
   };
   return execFileSync(npmCmd, args, { ...defaults, ...options }).trim();
 }