feat: scope-aware caching with session_id callback

[nogueira-raphael]

Summary

• Replace session_context_keys (CacheConfig field) with a session_id callback on the Ariadne adapter, following Apollo Server's scope-aware caching model
• Implement dual cache lookup: private key first (per-user), then public key (shared), so PUBLIC queries are never needlessly cached per-user
• PRIVATE responses without a session_id are skipped (not cached), with a warning logged
• Simplify Strawberry extension to public-only caching (context=None) until scope calculation is added

Test plan

• TestGetSessionId — callback returns value, returns None, no callback
• TestDualLookup — private hit, public hit after private miss, both miss, no sid skips private
• TestScopeAwareStore — PUBLIC stores with context=None, PRIVATE+sid stores with session context, PRIVATE without sid skips, not cacheable skips
• TestScopeAwareCaching (integration) — private per-user isolation, public shared across users
• Strawberry tests updated to assert context=None uniformly
• All 233 tests passing, mypy clean, ruff clean on changed files

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.74%. Comparing base (9898a62) to head (e284883).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
+ Coverage   88.10%   89.74%   +1.63%     
==========================================
  Files          33       33              
  Lines         925      926       +1     
  Branches      140      142       +2     
==========================================
+ Hits          815      831      +16     
+ Misses         87       71      -16     
- Partials       23       24       +1     

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