test: fix flaky tests_temp teardown cleanup

[knep]

Summary

Fixes the recurring CI flake in StreamingFormReaderTest (it broke PRs #21, #26 and #27, each needing a re-run). The failure was always at teardown:

OSError: [Errno 39] Directory not empty: 'tests_temp'
IsADirectoryError: [Errno 21] Is a directory: 'tests_temp'

Root cause — two bugs in test_utils._rmtree

1. Handler mishandled directories. The on_rm_error handler always called os.remove(path). But shutil.rmtree also invokes the handler for directories (when os.rmdir fails), and os.remove on a directory raises IsADirectoryError. Fixed by re-running the operation that actually failed — func(path) (os.unlink for files, os.rmdir for directories) — which is the pattern from the Python docs.

2. No tolerance for the race. Some tests upload files via background threads that are still writing into tests_temp when teardown starts; a file appearing between rmtree's scandir and rmdir leaves the directory non-empty. Added a retry loop (up to ~1s) so those threads can finish before giving up.

Testing

• New regression tests (tests/test_utils_rmtree_test.py): plain tree, read-only file (exercises the handler), and a concurrent writer (reproduces the race). 8/8 stable.
• Flaky test run 15× locally: 15/15 pass.
• Full backend suite: 1740 passed (×2 runs).
• No onerror DeprecationWarning on Python 3.14 (onerror kept for 3.10–3.11 compatibility; onexc is 3.12+).

🤖 Generated with Claude Code

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