Allow load_fixture to switch fixture path for already-loaded models

[danielahrnsbrak]

Summary

• TestHelper.load_fixture now unloads and reloads when called for an already-cached model with a different alternate_base_path. Previously, the second call was silently ignored regardless of path, leaving stale data from the first load.
• Calls with the same path remain a no-op (preserving the existing optimization).

Problem

When two different test setups call load_fixture for the same model class but with different fixture paths, the second call is silently skipped because the cache only checks @cache.key?(model_class) — it doesn't compare paths:

# Test A's setup:
FrozenRecord::TestHelper.load_fixture(ApiChange, "test/fixtures/webhooks")  # loads 27 records

# Test B's setup (different path, same model):
FrozenRecord::TestHelper.load_fixture(ApiChange, "test/fixtures/versioning")  # silently ignored — still 27 webhook records

This is a footgun in test suites where execution order is randomized and different test groups use different fixture directories for the same FrozenRecord model.

Fix

When load_fixture is called for a model already in the cache, compare the currently loaded base_path against the requested alternate_base_path. If they match, return early (no-op). If they differ, call unload_fixture first to restore the original base_path, then proceed with the new load. The original base_path (for unload_fixtures teardown) is preserved correctly because unload_fixture restores it before the re-load captures a new snapshot.

Tests

• Updated the existing "ignores repeat calls" test to specifically assert same-path no-op behavior
• Added test: switching to a different path loads the new fixture data
• Added test: unload_fixtures still restores the original base_path after path switches