Fix annotate-specification-links crash, CI trigger path, and UTF-8 file loading

[ApurveKaranwal]

Resolves #1190

Summary of Changes

This PR addresses three related tooling, cross-platform, and CI issues identified on the unify branch:

1. Fix TypeError and external template lookups in validation/annotate-specification-links:

   • Cast dialect to int(dialect) in applicable_dialects so checker.dialect_applies compares numeric versions against integers rather than strings.
   • Resolve external templates (rfc, iso, ecma262, perl5) directly from URLS["external"], which avoids false-positive missing template errors on future dialect placeholders ("compatibility": "9999").
   • Prevent in-place mutation of specification dictionaries (each = dict(each)).
   • Use .as_posix() for emitted annotation file paths to guarantee forward slashes across all operating systems.

2. Fix disconnected CI path filter in .github/workflows/show_specification_annotations.yml:

   • Update paths from 'tests/**' to 'validation/tests/**' so pull requests modifying the unified test suite properly trigger specification link checks.

3. Cross-platform UTF-8 file loading:

   • Add explicit encoding="utf-8" to path.read_text() and path.open() in check_validation_suite.py and annotate-specification-links to prevent UnicodeDecodeError on Windows environments defaulting to cp1252.

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Verification

• python validation/annotate-specification-links runs end-to-end and exits cleanly with code 0, emitting all notices with POSIX paths.
• python validation/check_validation_suite.py passes all 11 test suites across all 114 test files on Windows and POSIX systems.
• python -m py_compile validation/annotate-specification-links validation/check_validation_suite.py confirms no syntax errors.