Add comprehensive tests for cache_across_reloads gc behaviors

[Copilot]

This PR adds comprehensive test coverage for the garbage collection behavior of the cache_across_reloads decorator's memoization system.

Background

The cache_across_reloads decorator maintains two global dictionaries:

• memos: stores cached computation results keyed by (path, qualname)
• functions: stores function references keyed by (path, qualname)

The gc_memos() function, decorated with @post_reload, runs after module reloads to clean up orphaned memos for functions that no longer exist:

@post_reload
def gc_memos():
    for key in {*memos} - {*functions}:
        del memos[key]

This prevents memory leaks during hot reloading sessions when cached functions are removed or renamed.

Tests Added

Four new test functions comprehensively validate the garbage collection behavior:

1. test_cache_across_reloads_gc_function_removal: Verifies that memos are properly cleaned up when cached functions are completely removed from a module
2. test_cache_across_reloads_gc_function_modification: Ensures memos persist when functions are modified but not removed
3. test_cache_across_reloads_gc_selective_removal: Tests partial cleanup when some functions are removed while others remain
4. test_cache_across_reloads_gc_across_modules: Validates gc behavior across multiple modules

Each test directly inspects the memos and functions dictionaries to verify correct garbage collection behavior under various hot reload scenarios.

Fixes #348.

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