feat: add Node.js 26 support

[Poggen]

Summary

• add Node.js 26 to the supported engine range
• document Node.js 26 in README runtime compatibility
• add Node.js 26 to the CI test matrix

Closes #1355

Validation

• corepack yarn validate on Node.js v26.3.0
• 203 test suites passed, 4,464 tests passed, 1 skipped
• yarn lint:package passed with the existing publint suggestion only

Prompts used

User asked to research Auth0 node-auth0 Node.js 26 compatibility, then create an upstream issue and PR using the Poggen GitHub account.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.62%. Comparing base (57ae86c) to head (7bd1fb7).

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #1356    +/-   ##
========================================
  Coverage   89.62%   89.62%            
========================================
  Files         428      428            
  Lines       19886    19886            
  Branches     9400     9757   +357     
========================================
  Hits        17823    17823            
  Misses       2063     2063            

Flag	Coverage Δ
alltests	89.62% <ø> (ø)

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[developerkunal]

LGTM