test: isolate Django cache settings between xdist worker groups - #852

Summary

Fix Django cache settings leakage between parametrized test configurations when running pytest-xdist. Related to #777.

Changes

• Initialize a stable Django settings module and apply per-test cache configuration using override_settings.
• Deep-copy cache settings to prevent configuration changes from leaking between tests.
• Preserve configuration-specific connection factories and herd settings.
• Initialize the cache client while retaining Django's cache proxy.
• Correct cache fixture ordering for scan-iteration tests.
• Account for MessagePack's IntEnum serialization behavior.
• Skip sorted-set tests only for the unsupported ShardClient backend.
• Add regression tests covering cache configuration isolation, including Sentinel.

Validation

• Targeted regression tests with one worker.
• Full suite with 4 workers: 1,957 passed, 67 skipped.
• Full suite with 7 workers: 1,957 passed, 67 skipped.
• Ruff lint and formatting checks passed.
• Git diff validation passed.
• pre-commit.ci passed.
• GitHub Actions CI — awaiting upstream workflow authorization.

The skipped cases include 28 sorted-set tests for the unsupported sharded client; other backends retain their sorted-set coverage.

Related to #777.