🐛 fix(subprocess): stop hanging on undecodable backend output

[xiehuanyi]

A build backend, or a compiler it runs, can write bytes that the frontend's locale encoding cannot decode. Popen.communicate() decodes the captured output after the child exits, so it raises UnicodeDecodeError inside the output collection thread. Since #282 the frontend waits for that thread to store the output before it reads the backend result, so one bad byte leaves the frontend polling forever. 🐛 Before that change the same input crashed with a TypeError.

The frontend now decodes the backend's stdout and stderr with errors="backslashreplace". Those streams carry diagnostics; the backend result travels through a separate JSON file, so the lossy decode cannot touch it. Backslash escapes keep the original byte visible as before\xffafter, where replace would show a U+FFFD placeholder and hide which byte broke the log. 🔍

The request sent over stdin is ASCII JSON, so the error handler changes nothing on the encode side. An exception other than a decode error inside the output collection thread still hangs the frontend; this PR fixes the decode case.

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🟢 Approval recommended

The focused fix correctly prevents decoding failures, with regression coverage for both diagnostic streams.

0 open findings

🧠 Review effort: Balanced

[gaborbernat]