fix: Promise .then() must ignore non-function handlers per Promises/A+ 2.2.1

[MSNev]

When .then() receives a non-function, non-undefined handler, it incorrectly uses the handler value as the resolved value instead of passing through the settled value. This violates Promises/A+ spec section 2.2.1 which states that non-function handlers must be ignored.

Changes:

• Use isFunction(handler) check instead of isUndefined(handler) to determine whether to invoke the handler or pass through the settled value
• Use isFunction(handler) instead of truthy check to determine the resolve/ reject path for the chained promise
• Increase idle promise test timeout from 10s to 30s to account for requestIdleCallback deferral in headless browsers

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.49%. Comparing base (60a0710) to head (a95900d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #509      +/-   ##
==========================================
- Coverage   97.56%   97.49%   -0.08%     
==========================================
  Files          27       27              
  Lines        1439     1438       -1     
  Branches      336      338       +2     
==========================================
- Hits         1404     1402       -2     
- Misses         35       36       +1     

Files with missing lines	Coverage Δ
lib/src/promise/base.ts	98.98% <100.00%> (-0.26%)	⬇️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.