fix(cache): fail the run when a cache hit's outputs can't be restored

[wan9chi]

Motivation

When a cache hit's output archive can't be restored, for example because it was deleted or is corrupt, vp run prints an error but exits 0, and both the summary and --last-details report the task as a cache hit. CI passes without the task's outputs. Remote hits are also recorded locally before they're restored, so a remote hit that can't be restored leaves a local entry that fails every later run the same way, even though the remote entry is fine.

Changes

• A failed restore is saved in the summary as a new TaskResult::RestoreFailed. It counts as failed rather than as a hit, sets a non-zero exit code, and --last-details shows it as → Cache hit, but the outputs couldn't be restored (or Remote cache hit) followed by the error and its causes.
• For a local hit, the error is now Cache restore failed. Run `vp cache clean` to clear the cache: failed to extract the output archive: … instead of Cache lookup failed: failed to restore cached outputs from <path>; …. The entry stays in the cache, so the hint is still needed. For a remote hit, it's Cache restore failed: failed to extract the output archive: …, without the hint, because nothing was saved locally.
• A remote hit is saved locally only after its outputs are restored, so a failed restore just deletes the downloaded archive, and the next run fetches the remote entry again.
• Tests: output_cache_test deletes the archive of a cached task and checks the failed hit, --last-details, and that the task executes again after vp cache clean. This runs on all platforms. remote_cache::restore_failure (ignored and not on Windows, like the other remote backend cases) makes a remote hit's restore fail and checks that the downloaded archive is removed and the next run fetches and restores the entry again. vtt rm --ext <suffix> <dir> removes the archive without hardcoding its name or the cache schema directory.

Closes #767

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.50%  [ -6.00% ..  +8.82%]  overhead  +269.29%
dynamic/access             change  -0.15%  [ -1.60% ..  +1.85%]  overhead   +13.91%
dynamic/access-relative    change  -0.22%  [ -1.74% ..  +2.21%]  overhead   +60.53%
dynamic/access-contended   change  -0.69%  [ -3.76% ..  +1.39%]  overhead   +14.75%
static/launch              change  +1.99%  [ -4.32% ..  +7.33%]  overhead  +727.87%
static/access              change  -0.64%  [ -1.90% ..  +0.90%]  overhead  +808.63%
static/access-relative     change  +0.17%  [ -0.89% ..  +1.16%]  overhead +1385.11%
static/access-contended    change  -0.21%  [ -1.13% ..  +0.73%]  overhead +3162.54%

macos

dynamic/launch             change  -0.01%  [-10.67% .. +11.09%]  overhead  +238.39%
dynamic/access             change  +2.01%  [-13.42% .. +89.12%]  overhead    +8.45%
dynamic/access-relative    change  +0.74%  [ -6.43% ..  +6.72%]  overhead  +241.79%
dynamic/access-contended   change  +0.64%  [ -6.09% .. +14.89%]  overhead    +1.05%

windows

dynamic/launch             change  +0.35%  [ -5.37% ..  +8.05%]  overhead   +25.21%
dynamic/access             change  -0.51%  [-18.23% ..  +8.92%]  overhead    +1.74%
dynamic/access-relative    change  -1.86%  [-35.22% ..  +0.51%]  overhead    +1.19%
dynamic/access-contended   change  -0.89%  [-11.45% ..  +6.09%]  overhead    +2.20%