test: fix flaky PTY env test

[knep]

Summary

Stabilises process_pty_test.py::TestEnvironmentVariables::test_PYTHONUNBUFFERED, which intermittently failed in CI (most recently on Python 3.14, PR #29) with:

AssertionError: '1' != None

i.e. the PYTHONUNBUFFERED line was missing from the captured environment.

Root cause — a read race against the output stream

start() reads the process output on a separate thread that pushes chunks into a ReplayObservable and close()s it when done. The tests read via read_until_closed() while that thread was still pushing, after only a thread.join(timeout=0.1).

ReplayObservable.subscribe() (replay buffered chunks, then register the observer) is not synchronised against push() (append chunk, fire to observers). A chunk pushed in the window between "replay finished" and "observer registered" is neither replayed nor delivered live — so the last line of output could be dropped. The tiny, fast printenv.sh output made this most likely, and fast/loaded runners (3.14) hit it.

Fix (test-only)

Wait for output_stream.wait_close() before reading. Once the stream is closed there is no concurrent pusher, and close() keeps the replay buffer (only dispose() clears it, which these tests don't call), so the late subscribe replays every chunk deterministically. Extracted as a shared _run_and_read_all() helper and applied to the env tests and the unicode/encoding tests (which had the same latent race).

No production code changed.

Testing

• process_pty_test.py run 20× locally: 20/20 pass (and the fix is deterministic by construction, not just lucky).
• Full backend suite: 1740 passed (×2 runs).

Follow-up (out of scope)

The underlying ReplayObservable.subscribe/push race is a real (if rare) concurrency gap that could also affect a production subscriber connecting mid-execution. Hardening it would mean adding synchronisation to that core class — deliberately left out of this test-only fix.

🤖 Generated with Claude Code

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