refactor(extensions): introduce argument matcher

[benbellick]

Function variant matching currently spreads argument position resolution, nullability handling, variadic expansion, and constrained any validation across several helpers. This refactors those responsibilities behind an internal matcher so the matching flow has one place to evolve without changing the exported matching APIs.

This is intended to be behavior-preserving groundwork for improving constrained any matching semantics in a follow-up PR.

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Note: This PR was developed with AI assistance. All changes have been reviewed, and I take full responsibility for this contribution.

[codecov]

Codecov Report

❌ Patch coverage is 93.61702% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.28%. Comparing base (0ae704f) to head (36fb338).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
extensions/variants.go	93.61%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #245      +/-   ##
==========================================
+ Coverage   68.85%   69.28%   +0.43%     
==========================================
  Files          47       47              
  Lines       10866    10980     +114     
==========================================
+ Hits         7482     7608     +126     
+ Misses       3030     3009      -21     
- Partials      354      363       +9     

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