Fix cache entries being lost when multiple tests share cassette

[ab-10]

Summary

Fixed a bug where cache entries from one test would be lost when another test using the same cassette file ran in RECORD_MODE=all. The issue occurred because each cache instance would clear existing entries instead of preserving them.

Changes

• Cache initialization in ALL mode: Changed to load existing entries before recording, rather than clearing them. This allows multiple test instances to accumulate entries in the same cassette file.
• Entry replacement logic: Modified put() method to replace existing entries with the same instruction and method (supporting re-recording) instead of always appending, preventing duplicates.
• Comprehensive test coverage: Added 210 lines of unit tests covering:
  • Multiple cache instances preserving each other's entries
  • Re-recording replacing existing entries instead of duplicating
  • Different methods being treated as separate entries
  • Behavior across all record modes (ONCE, NONE, ALL)

Implementation Details

• When RECORD_MODE=all and a cassette exists, cache._load() is now called to preserve existing entries before recording new ones
• The put() method now checks for existing entries matching both instruction and method, replacing them in-place rather than appending
• Entry replacement triggers an immediate save() to persist changes
• Tests use temporary directories and proper environment variable cleanup to avoid side effects

https://claude.ai/code/session_01PXyXZNXxqTusmww6BdzqkF