fix(types): align Response with DOM fetch types

[theamodhshetty]

Summary

• align SpecIterableIterator with the iterator-helper surface expected by modern DOM HeadersIterator types
• add the missing File[Symbol.toStringTag] declaration on node:buffer so Response.formData() stays structurally compatible
• add a tsd regression proving Undici Response is assignable to globalThis.Response

Why

Newer TypeScript DOM libs model Headers.entries()/keys()/values() as HeadersIterator, which includes iterator helper methods. Undici's SpecIterableIterator was missing that surface, so undici.Response could not be used where a standard Response type was expected. Once that was fixed, formData() still diverged because Node's buffer.File typing was missing the Symbol.toStringTag property that exists at runtime.

Tests

• npm run test:typescript
• npm run lint

Fixes #3639

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.14%. Comparing base (bfeacd0) to head (7d5ad97).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4867   +/-   ##
=======================================
  Coverage   93.14%   93.14%           
=======================================
  Files         109      109           
  Lines       34254    34254           
=======================================
  Hits        31905    31905           
  Misses       2349     2349           

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[mcollina]

lgtm