quic: preserve session stats after close

[manNomi]

Description

QuicSessionStats[kFinishClose]() used IDX_STATS_STREAM_COUNT when
snapshotting session statistics. Since stream stats contain fewer entries
than session stats, fields beginning with cwnd were omitted after a session
was destroyed and returned undefined.

Use IDX_STATS_SESSION_COUNT so the complete session stats slice is copied
before the native session releases its arena slot. Add regression assertions
for the first previously omitted field (cwnd) and the final session field
(streamsIdleTimedOut).

[nodejs-github-bot]

Review requested:

• @nodejs/quic

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.25%. Comparing base (ed63b19) to head (7a25eb7).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64489      +/-   ##
==========================================
- Coverage   92.01%   90.25%   -1.77%     
==========================================
  Files         381      741     +360     
  Lines      170167   241688   +71521     
  Branches    26075    45549   +19474     
==========================================
+ Hits       156585   218139   +61554     
- Misses      13291    15102    +1811     
- Partials      291     8447    +8156     

Files with missing lines	Coverage Δ
lib/internal/quic/stats.js	100.00% <100.00%> (ø)

... and 486 files with indirect coverage changes

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[bjohansebas]

LGTM!