[7.x] fix: support Node 26 and legacy global dispatcher

[mcollina]

Summary

Backport Node 26 support and the legacy global dispatcher mirroring added in undici 8.

Changes

• add Node 26 to the CI matrices
• run test/node-test through a dedicated diagnostic runner on Node 26
• tolerate the retried POST ordering seen on Node 26 in test/node-test/client-errors.js
• isolate the current global dispatcher on undici.globalDispatcher.2
• mirror a legacy-compatible dispatcher on undici.globalDispatcher.1
• export Dispatcher1Wrapper and add typings/tests for it
• force allowH2: false for legacy v1 dispatcher consumers
• add node tests covering the global dispatcher compatibility behavior

Testing

• node --test test/node-test/global-dispatcher-version.js
• node --test test/node-test/client-errors.js
• npm run test:typescript
• npx eslint index.js lib/global.js lib/dispatcher/dispatcher1-wrapper.js test/node-test/global-dispatcher-version.js test/node-test/client-errors.js scripts/run-node-test-ci.mjs
• node scripts/run-node-test-ci.mjs

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.29197% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.84%. Comparing base (2b3cd22) to head (35020f2).

Files with missing lines	Patch %	Lines
lib/dispatcher/dispatcher1-wrapper.js	77.86%	27 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             v7.x    #5319      +/-   ##
==========================================
- Coverage   92.89%   92.84%   -0.06%     
==========================================
  Files         112      113       +1     
  Lines       35828    35964     +136     
==========================================
+ Hits        33283    33391     +108     
- Misses       2545     2573      +28     

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