fix(parse): pause req during async writeHeaders to avoid chunk loss

[guimard]

Summary

Since parse() was made async in 3.3.2, it awaits writeHeaders() before attaching its data handling (now req.pipe(pipe) after #1017). If a caller has attached a 'data' listener on the request before calling form.parse() — a common pattern for content-length accounting or logging — the request stream is already in flowing mode. Chunks that arrive during the microtask yielded by the await are emitted and lost before Formidable pipes the stream, causing the first part(s) of a multipart body to be silently dropped.

Reproduction

See the added test test-node/standalone/parse-data-race.test.js. A server attaches req.on('data', …) for content-length accounting, then calls form.parse(req, cb) with a multipart body containing three fields. Without the fix, only the last field is parsed; with the fix all three arrive.

The regression was originally reported downstream via node-form-data's test-ranged-filestream.js, where a req.on('data') listener for content-length verification causes the first two of five parts to vanish.

Fix

Pause the request at the top of parse() so chunks are buffered in the request's internal queue until the parser is ready. req.pipe(pipe) later resumes the stream, so no explicit resume() is needed.

Test plan

• New regression test test-node/standalone/parse-data-race.test.js passes with the fix and fails without it
• Existing test suite still passes