fix: preserve HTTP/2 for non-upgrade legacy requests

[mcollina]

Summary

• restrict the legacy dispatcher HTTP/1.1 compatibility override to upgrade requests
• allow Node.js built-in fetch() to negotiate HTTP/2 through an Undici v8 global dispatcher
• keep the native WebSocket fallback on HTTP/1.1 and assert its negotiated protocol
• add an HTTP/2-only server regression fixture for built-in fetch()

Why

Dispatcher1Wrapper currently forces allowH2: false for every request. On Node.js versions whose built-in fetch consumes undici.globalDispatcher.1, this prevents a configured Undici v8 dispatcher from reaching servers that only advertise h2 through ALPN.

The override was introduced for legacy WebSocket consumers that cannot retry with HTTP/1.1 when HTTP/2 Extended CONNECT is unavailable. Scoping it to upgrade requests preserves that workaround without changing the protocol for ordinary fetch traffic.

Refs: #4989 and #4990.

Tests

• npm run lint
• npx borp -p "test/node-test/global-dispatcher-version.js" on Node.js 22.22.1, 24.18.1, and 26.8.1
• npx borp -p "test/websocket/issue-4989.js" on Node.js 22.22.1, 24.18.1, and 26.8.1
• npm run test:unit (one unrelated high-concurrency failure in test/issue-4880.js; isolated rerun passed)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.48%. Comparing base (cb7373a) to head (584ca8b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5811      +/-   ##
==========================================
- Coverage   93.52%   93.48%   -0.05%     
==========================================
  Files         110      110              
  Lines       39415    39522     +107     
==========================================
+ Hits        36864    36947      +83     
- Misses       2551     2575      +24     

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