fix(h2): avoid pool fan-out during ALPN negotiation

[mcollina]

Fixes #4143.

Delay Pool connection fan-out while an HTTP/2-capable client is negotiating TLS/ALPN. Concurrent requests remain queued behind one protocol probe; after negotiation they multiplex over HTTP/2, while HTTP/1.1 restores normal connection fan-out.

This also covers repeated cold bursts after idle session reaping, preserves HTTP/1.1 concurrency, and updates the Agent and Pool documentation.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.26174% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.50%. Comparing base (cb7373a) to head (7a64696).

Files with missing lines	Patch %	Lines
lib/dispatcher/pool-base.js	77.96%	13 Missing ⚠️
lib/dispatcher/pool.js	96.66%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5799      +/-   ##
==========================================
- Coverage   93.52%   93.50%   -0.03%     
==========================================
  Files         110      110              
  Lines       39415    39555     +140     
==========================================
+ Hits        36864    36986     +122     
- Misses       2551     2569      +18     

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