perf(user_api): implement centralized caching for user preferences

[andrey-canon]

Description

This PR introduces a centralized caching mechanism for the User Preferences API to mitigate severe database load and reduce redundant queries across the platform.

Previously, frequent user preference lookups were causing high operational impact, specifically driven by the user preference lookup query.

SELECT `user_api_userpreference`.`id`, `user_api_userpreference`.`user_id`, `user_api_userpreference`.`key`, `user_api_userpreference`.`value` FROM `user_api_userpreference` WHERE (`user_api_userpreference`.`key` = ? AND `user_api_userpreference`.`user_id` = ?) LIMIT ?

By centralizing the cache at the Model layer using Django's standard caching framework, this PR stabilizes the database, improves global cache utilization, and simplifies the API layer.

Key Changes

• Model-Layer Caching: Removed the @request_cached() decorator from UserPreference.get_all_preferences() in models.py. The cache Hit/Miss logic is now explicitly handled inside this method.
• Centralized Cache Helpers: Extracted cache key generation, get, set, and invalidate methods into openedx/core/djangoapps/user_api/helpers.py.
• API Simplification: Cleaned up api.py. Methods like has_user_preference, get_user_preference, and get_user_preferences now directly delegate to the model layer rather than handling caching themselves.

Testing & Impact

• Reduced Query Counts: Successfully reduced expected database queries for preference lookups (e.g., expected queries in lang_pref/tests/test_middleware.py dropped from 1 to 0 and 3 to 2; taxonomy tags test queries reduced by 1).
• Test Environment Updates: Added @override_settings(CACHES={'default': {'BACKEND': 'django.core.cache.backends.locmem.LocMemCache'}}) to affected test cases (like IndexQueryTestCase). Since the architecture no longer relies on thread-local memory, tests that previously defaulted to DummyCache now require a functional cache backend to accurately assert the optimized query counts.

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