Respect node namespace context in isDefaultNamespace

[olavoasantos]

Summary

• Respect node namespace context in isDefaultNamespace.
• Add focused regression coverage for the corrected behavior.

Stack

• Stack: Namespace and selectors (#698), layer 1 of 3
• Base: normalize-dom-namespaces
• This is the bottom of its stack.
• This stack depends on Core foundation PR Normalize DOM names and attribute namespaces #677.

Validation

Fresh GitHub CI on restacked head 5100d3f passes:

• lint;
• type-check and build;
• unit tests with coverage;
• bundle-size checks;
• Playwright end-to-end tests;
• the classified Web Platform Test suite.