http: fix drain event with cork/uncork in ServerResponse

[ThierryMT]

Fixes #60432

When using cork() and uncork() with ServerResponse, the drain event was not reliably emitted after uncorking. This occurred because the uncork() method did not check if a drain was pending (kNeedDrain flag) after flushing the chunked buffer.

The Problem:

• When corked, write() buffers data in kChunkedBuffer instead of sending immediately
• If write() returns false, kNeedDrain is set to true
• When uncork() is called, it flushes the buffer by calling _send()
• After flushing, uncork() never checked if drain was needed or emitted the event
• This left the response waiting for a drain event that would never fire

The Fix:
This commit ensures that when uncork() successfully flushes buffered data and a drain was needed, the drain event is emitted immediately.

Changes:

• Modified OutgoingMessage.prototype.uncork() in lib/_http_outgoing.js
• Capture return value from final _send() call
• Check if drain was pending after successful flush
• Emit drain event when appropriate

Testing:
The issue could be reproduced by using cork() with multiple large writes and checking for drain events. With this fix, the drain event now fires correctly after uncork().

[nodejs-github-bot]

Review requested:

• @nodejs/http
• @nodejs/net

[mcollina]

lgtm

[mcollina]

@ThierryMT can you add a test?

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/69906/

[pimterry]

Fix LGTM 👍

Test looks good if we can avoid the branch, but there are some lint warnings here on the test JS & the commit message that will need fixing before we can merge this.

[pimterry]

This should always need drain right? It's corked so there's no race condition or anything.

Unless there's a good reason, imo it would be better to enforce that (assert.strictEqual(res.write('2'.repeat(1000000)), false)) and drop the if here entirely.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.57%. Comparing base (d9cf867) to head (9ab4cc4).
⚠️ Report is 55 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #60437      +/-   ##
==========================================
- Coverage   88.58%   88.57%   -0.02%     
==========================================
  Files         704      704              
  Lines      207815   207832      +17     
  Branches    40036    40042       +6     
==========================================
- Hits       184102   184096       -6     
- Misses      15758    15785      +27     
+ Partials     7955     7951       -4     

Files with missing lines	Coverage Δ
lib/_http_outgoing.js	97.68% <100.00%> (+2.00%)	⬆️

... and 31 files with indirect coverage changes

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[Ethan-Arrowood]

this looks good. can you fix the formatting in the test file?