fix: FIT-1054: Ensure redis cache is used if available for all FSM operations which use cache

[bmartel]

This pull request updates the caching mechanism used by the FSM (Finite State Machine) state management in label_studio/fsm/state_manager.py. The main goal is to ensure that FSM operations use the correct cache backend, especially in environments where the default cache may be a dummy cache (e.g., for testing or specific deployments), and to support using a dedicated Redis cache if configured. The changes also update tests to ensure cache isolation.

Caching backend improvements:

• Introduced the get_fsm_cache() function to select the appropriate cache backend for FSM operations, preferring a Redis cache if specified via REDIS_CACHE_ALIAS in settings, and falling back to the default cache otherwise. This ensures FSM caching works correctly even if the default cache is a dummy backend. The cache backend is memoized for efficiency.
• Replaced all direct uses of cache in FSM state management methods with get_fsm_cache(), ensuring consistent use of the selected cache backend for all FSM cache operations, including get, set, add, delete, and set_many. [1] [2] [3] [4] [5] [6] [7]

Testing improvements:

• Updated the FSM test fixture to clear both the default cache and the FSM-specific cache after each test, ensuring proper test isolation regardless of which cache backend is in use. [1] [2]

Imports and setup:

• Updated imports to include caches from django.core.cache and import get_fsm_cache in tests. [1] [2]

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[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.03%. Comparing base (a2f4b57) to head (dac1a19).
⚠️ Report is 1 commits behind head on develop.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
label_studio/fsm/state_manager.py	93.33%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #8896       +/-   ##
============================================
+ Coverage    66.10%   81.03%   +14.92%     
============================================
  Files          820      259      -561     
  Lines        64416    23561    -40855     
  Branches     11041        0    -11041     
============================================
- Hits         42581    19092    -23489     
+ Misses       21831     4469    -17362     
+ Partials         4        0        -4     

Flag	Coverage Δ
lsf-e2e	?
lsf-integration	?
lsf-unit	?
pytests	81.03% <93.75%> (+<0.01%)	⬆️

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