Fix cache miss in replay mode for subsequent tests

[recursiveAF]

Summary

• Fix bug where first test passes but subsequent tests fail with "Cache miss in replay mode"
• ONCE mode now replays existing entries AND records new ones on cache miss
• Added comprehensive test suite for all three recording modes (ONCE, NONE, ALL)

Problem

When running multiple tests in a pytest session (or adding new tests in subsequent sessions), the second test would fail with:

AssertionError: Flow.expect failed
Expected: world
Reason: Assertion generation/execution failed: Cache miss in replay mode: expect() with instruction: 'world'

Root Cause

In ONCE mode, after the first test created the cassette file, subsequent Cache.from_env() calls would see the cassette exists and switch to replay-only mode (_should_record=False), causing any new instructions to fail instead of being recorded.

Solution

Changed ONCE mode semantics to always allow recording while also loading existing entries:

• Before: Record if cassette missing, replay-only if exists
• After: Replay existing entries, record new ones on cache miss

This is the expected behavior for a "record once" mode - it should accumulate entries over time, not lock after the first test.

Test plan

• Added 15 unit tests covering all recording modes
• pytest tests/test_cache_modes.py -v passes

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