Claude Sonnet 5.5 (effort default (no --effort flag) · thinking on) on Fix a flaky test suite caused by set ordering, ignored date parameter and global random state - without touching the tests

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  "result": "I rewrote `billing.py` so it no longer depends on the clock, the hash seed or the global `random` state. pytest isn't installed here, so I called the three test functions directly under `PYTHONHASHSEED` 0, 1 and 2. All passed each time. I didn't test other dates or confirm that different months give different samples.\n\nWhat was causing the flakiness:\n- **Ignored `today`:** the function overwrote the argument with `date.today()`. That is why the due-date test failed on certain days. It now uses the current date only when `today` isn't passed.\n- **Unordered customers:** customers were iterated from a `set`, so invoice order and numbering changed with the hash seed. They are now sorted case-insensitively, with ties broken by the exact name, and numbered in that order.\n- **Global random state:** the audit sample came from the global `random`. It now comes from a private `random.Random` seeded with a SHA-256 hash of the date, `audit_size` and the invoice customers and totals. The same inputs always give the same sample, the global state is left untouched, and the sample still looks random.\n\nThe seed uses the full date rather than just the month, so two runs in the same month on different days can pick different samples. I left `tests/test_billing.py` and the function signature unchanged.",
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