feat: context-aware cache keys (session_context_keys)

[nogueira-raphael]

Summary

• Add session_context_keys field to CacheConfig — specifies which keys from the GraphQL context dict are included in cache key generation
• Ariadne and Strawberry adapters extract configured keys and pass them to the cache service, producing per-user cache entries
• Fix: Ariadne handler now resolves context_value via get_context_for_request() before cache lookup (previously it was None at that point, so all users shared the same cache key)
• Add fake token auth (Authorization: Bearer user-{id}) to the Ariadne example to enable per-user PRIVATE cache testing

Closes #12

Changes

• src/cacheql/core/entities/cache_config.py — new session_context_keys: list[str] | None field
• src/cacheql/adapters/ariadne/handler.py — _extract_session_context(), early context resolution, pass context= to cache service
• src/cacheql/adapters/strawberry/extension.py — same pattern for Strawberry adapter
• examples/fastapi-ariadne-redis/ — fake token auth + session_context_keys=["current_user_id"]
• tests/ — unit tests for context extraction (both adapters), cache service context isolation, integration test for per-user cache isolation

Test plan

• make typecheck — no issues
• 229 tests pass (make test)
• Start the Ariadne example and verify:
  • me with Bearer user-1 → Alice (cached per user 1)
  • me with Bearer user-2 → Bob (different cache entry)
  • Same user gets cache HIT on second request

[codecov]

Codecov Report

❌ Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.10%. Comparing base (fb88ebc) to head (cc1340f).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/cacheql/adapters/ariadne/handler.py	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #24      +/-   ##
==========================================
+ Coverage   87.94%   88.10%   +0.16%     
==========================================
  Files          33       33              
  Lines         904      925      +21     
  Branches      136      140       +4     
==========================================
+ Hits          795      815      +20     
  Misses         87       87              
- Partials       22       23       +1     

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