[ByteJuggler]

Fixes #2159.

get_prebuilt moves BUILD.json to BUILD.json.prev before downloading, and update_installed_engine only puts it back if the process survives to its new is None branch. After a crash, reboot or closed window mid-update, every later update returned early on the missing BUILD.json, so the install stayed on its old engine without saying so.

Now, when BUILD.json is missing but BUILD.json.prev and the exe are there, the stamp is restored (with a warning) and the update runs again.

• New test test_an_interrupted_update_is_tried_again (fails before the change, passes after).
• tools/ suite: 3 failures in test_setup_engine_hash.WhatARefusalDoesToTheCaller, which fail the same way on main without this change.

[qiuzan2001]

Checked on Linux: tools/test_setup_stale_stamp.py passes here and fails against main's setup.py, so the toolkit-13 case is fixed. The experimental CUDA 12 engine is still left out: the toolkit is None branch of update_installed_engine (setup.py:3090) visits engine-cuda12/ only when engine_dir(12) / "BUILD.json" exists, and after an interrupted update of that engine only BUILD.json.prev is there, so the new restore code is never reached for it. With both engine/ and engine-cuda12/ holding only an exe and BUILD.json.prev, engine/ gets its stamp back and engine-cuda12/ does not. Accepting BUILD.json.prev in that gate (and a toolkit-12 case in the test) would cover it.

[Niko1221]

Thanks for the pull request.

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