refactor: remove deprecated CacheExtension from Ariadne adapter

[nogueira-raphael]

Summary

• Remove CacheExtension class and create_cache_extension function from the Ariadne adapter
• Remove src/cacheql/adapters/ariadne/extension.py entirely
• Remove 9 associated integration tests (TestCacheExtension, TestCacheExtensionWithVariables)
• Update src/cacheql/__init__.py docstring to reference CachingGraphQL instead of CacheExtension

CacheExtension was deprecated because Ariadne's extension system is synchronous and doesn't properly await async cache operations. CachingGraphQL is the recommended replacement.

Test plan

• All 130 tests passing (9 removed with the class)
• mypy clean
• Verified no production code depends on CacheExtension (Strawberry's CacheExtension is unaffected)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.64%. Comparing base (94830c5) to head (8d55dff).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
+ Coverage   79.12%   80.64%   +1.51%     
==========================================
  Files          34       33       -1     
  Lines        1025      904     -121     
  Branches      162      136      -26     
==========================================
- Hits          811      729      -82     
+ Misses        175      152      -23     
+ Partials       39       23      -16     

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