fix: reap idle HTTP/2 sessions

[mcollina]

Summary

• add an idle timeout for HTTP/2 sessions so keepAliveTimeout is honored after streams complete
• clear the idle timer when new streams start and when the session is closed/detached
• add a regression test for HTTP/2 sessions are never idle-reaped: keepAliveTimeout has no effect when h2 is negotiated (default since v8) #5381

Fixes: #5381

Tests

• npx borp -p test/issue-5381.js
• npx borp -p "test/http2-*.js"
• npm run lint -- --fix-dry-run

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.72131% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.24%. Comparing base (ee59da3) to head (812ff07).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/dispatcher/client-h2.js	96.72%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5406      +/-   ##
==========================================
- Coverage   93.25%   93.24%   -0.01%     
==========================================
  Files         110      110              
  Lines       36752    36885     +133     
==========================================
+ Hits        34274    34395     +121     
- Misses       2478     2490      +12     

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