test: add Node.js global fetch regression coverage

[mcollina]

Description

Add regression coverage for the Node.js built-in fetch + Undici Agent combination discussed in #5341.

This does not change runtime behavior on main; it just locks in that response headers remain visible and compressed bodies continue to decode correctly when fetch() is used with:

• the default global dispatcher
• an explicit dispatcher: new Agent()

The new test is skipped on Node.js versions before 26, since the specific built-in fetch/dispatcher bridge path under test is the Node 26 behavior.

Testing

• npm run lint -- test/node-test/global-dispatcher-version.js
• npx borp -p "test/node-test/global-dispatcher-version.js"
• source ~/.nvm/nvm.sh && nvm use 26.3.0 >/dev/null && npx borp -p "test/node-test/global-dispatcher-version.js"

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.24%. Comparing base (9b1d58f) to head (430ba5d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5361   +/-   ##
=======================================
  Coverage   93.23%   93.24%           
=======================================
  Files         110      110           
  Lines       36668    36719   +51     
=======================================
+ Hits        34187    34238   +51     
  Misses       2481     2481           

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