test(redis cache): re-enable Test_RedisScheme + not-installed test for redis-py 8.x

[wbarnha]

Description

Re-enables three redis cache tests skipped "Needs fixing" that had drifted against redis-py 8.x and the current cache backend.

• Test_RedisScheme::test_single_client / test_cluster_client referenced redis.StrictRedis / aredis.RedisCluster as attributes of the backend module — which no longer exist there. They now assert against the scheme→client-class map the backend actually builds (_client_by_scheme): single-node resolves to redis-py 8.x's concrete redis.client.Redis; cluster resolves to redis.RedisCluster. The single-node client is built for real (redis-py connects lazily) and its connection_pool.connection_kwargs (host/port/db) are checked; the cluster class — which connects on construction — is mocked exactly like the existing mocked_redis fixture, while scheme selection and db-stripping run for real. test_single_client is strictly stronger than the original.
• test_redis__aredis_is_not_installed now patches the redis module name the backend's on_start guard actually reads (if redis is None), so it raises ImproperlyConfigured without opening a socket (the old fixture patched aredis, which the guard never checks — so it opened a real connection to localhost:6079).

Only external redis client classes are mocked; the backend logic runs for real. Test-only change.

Latent source bug noticed (not fixed here)

The backend does import redis.asyncio as aredis but its on_start guard only checks if redis is None, never aredis. So the "async redis not installed" path is effectively dead / guards the wrong name. Left for a separate source-side PR to keep this change test-only.

Verification

tests/functional/web/test_cache.py → 32 passed; flake8/black/isort clean. Produced under adversarial faithfulness review.

🤖 Generated with Claude Code

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.19%. Comparing base (0a92897) to head (23a912f).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #721      +/-   ##
==========================================
+ Coverage   94.15%   94.19%   +0.04%     
==========================================
  Files         104      104              
  Lines       11136    11136              
  Branches     1201     1201              
==========================================
+ Hits        10485    10490       +5     
+ Misses        550      547       -3     
+ Partials      101       99       -2     

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[wbarnha]

Closing in favour of #724, which now contains this test rewrite plus the backend not-installed-guard source fix, as a single independent PR off master. No content is lost — it all lives in #724.

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