fix(h2): allow stream-bodied requests to multiplex on a busy session

[GiHoon1123]

This relates to...

Fixes #5524
Fixes #5494

Same root cause for both -- #5494 already has an open fix in #5497 (same core change to busy(), currently blocked on a requested unit test) -- see "Relationship to #5497" below before reviewing.

Rationale

busy() in lib/dispatcher/client-h2.js deferred any request with a stream/async-iterable/FormData body until every other in-flight request on the h2 session completed. If one of those in-flight requests is a long-lived stream (e.g. an open text/event-stream GET), every bodied request queued behind it parks indefinitely. The client also stops reporting kBusy while an h2 context is attached (so the pool keeps multiplexing instead of opening a second connection), so there's no escape route -- for an SSE stream, the hang is permanent.

The two hazards the guard's comments cited are already handled more precisely elsewhere in this file:

• "can error while other requests are inflight and indirectly error those as well" -- a stream erroring mid-request now only aborts that one stream via the abort() closure (explicit comment there: "We do not destroy the socket as we can continue using the session"); it doesn't touch sibling streams.
• "cannot be retried... could cause failure" -- canRetryRequestAfterGoAway() already excludes stream/iterable/FormData bodies from the requests it resurrects after a GOAWAY (body == null || isBuffer || isBlobLike only), erroring the rest instead of replaying a partially-consumed body.

Both of those were already in place before this change, so the blanket pre-dispatch guard was redundant.

Relationship to #5497

#5497 removes the same 9 lines for the same reason, opened against #5494 (concurrent POSTs on h2 not running concurrently -- a throughput complaint). #5524 is a more severe symptom of the identical guard: not just serialized, but a permanent deadlock when one of the concurrent requests is a long-lived stream like SSE. I only found #5497 after finishing this fix independently.

This PR adds what #5497 doesn't currently have:

• The unit test mcollina requested on feat(h2): enable concurrency on idempotent requests #5497 (still outstanding as of this PR) -- covering HTTP2 POST requests over fetch() to the same origin can't run concurrently #5494's own scenario directly: a second stream-bodied POST now dispatches while the first is still in flight instead of waiting for it to complete.
• A separate regression test for the more severe HTTP/2: fetch POST hangs while an SSE stream is open #5524 deadlock scenario (SSE held open + bodied POST hangs forever, not just serializes).

Happy to close this in favor of #5497 if the maintainers would rather continue there -- opening it mainly so the missing test and both scenarios' coverage aren't lost. No preference on which one lands.

Changes

Features

N/A

Bug Fixes

• Fixes HTTP/2: fetch POST hangs while an SSE stream is open #5524: a bodied fetch()/request() call on an h2 session no longer waits forever behind an open long-lived stream (e.g. SSE) on the same session.
• Fixes HTTP2 POST requests over fetch() to the same origin can't run concurrently #5494: concurrent bodied fetch()/request() calls on the same h2 session now dispatch concurrently instead of serializing.

Breaking Changes and Deprecations

N/A -- this only removes a guard that prevented dispatch; nothing that previously worked changes behavior, only what could not previously proceed now can.

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

[mcollina]

lgtm

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.44%. Comparing base (3c662a5) to head (46eda23).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5538      +/-   ##
==========================================
- Coverage   93.44%   93.44%   -0.01%     
==========================================
  Files         110      110              
  Lines       37329    37319      -10     
==========================================
- Hits        34883    34871      -12     
- Misses       2446     2448       +2     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.