fix(http2): respect peer max concurrent streams

[trivikr]

This relates to...

Fixes: #5134

Rationale

HTTP/2 remote settings update client[kMaxConcurrentStreams], but dispatch was only throttled by pipelining.

When pipelining was higher than the server's advertised stream limit, Undici could open too many concurrent streams and receive NGHTTP2_REFUSED_STREAM.

Changes

• Queue HTTP/2 requests while remote SETTINGS have not arrived and one stream is already running.
• Queue HTTP/2 requests when the number of running streams reaches client[kMaxConcurrentStreams].

Features

N/A

Bug Fixes

Respect http/2 peer max concurrent streams

Breaking Changes and Deprecations

N/A

Status

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

Assisted-by: openai:gpt-5.5

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.16%. Comparing base (aeed21a) to head (73325dd).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5135   +/-   ##
=======================================
  Coverage   93.15%   93.16%           
=======================================
  Files         110      110           
  Lines       36212    36220    +8     
=======================================
+ Hits        33735    33743    +8     
  Misses       2477     2477           

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[mcollina]

lgtm