fix(managers): utilize prefetch cache in names() and slugs() (#936)

[SparshGarg999]

Fixes #936

Problem

Calling .names() or .slugs() on a _TaggableManager on model instances loaded via prefetch_related("tags") executed new SQL queries (values_list()) instead of leveraging the prefetched tags stored in memory in _prefetched_objects_cache. This resulted in unexpected N+1 query loops despite prefetching.

Solution

• Updated names() and slugs() in _TaggableManager (taggit/managers.py) to inspect _prefetched_objects_cache on the instance and extract tag names/slugs directly from cached tag instances if available, avoiding extra DB queries.
• Retained standard queryset values_list() evaluation when prefetch cache is not present.
• Added unit test test_names_and_slugs_use_prefetch_cache to TestPrefetchCache in tests/test_models.py verifying 0 SQL queries are executed.