fix: repair toolchains missing their installed manifest

[ychampion]

Summary

• fetch the channel manifest when the installed v2 manifest is missing
• repair damaged toolchains through both explicit install and all-channel update paths

Why

A matching update hash could skip the channel manifest download even after the installed manifest was deleted, leaving the toolchain broken while rustup reported it as unchanged.

Fixes #4825.

Validation

• cargo test --features=test --test test_bonanza suite::cli_rustup:: - 130 passed
• cargo test --features=test --lib v2_manifest_checksum_mismatch_surfaces_error - passed
• cargo clippy --features=test --all-targets -- -D warnings - passed
• cargo fmt --all -- --check - passed

[djc]

Please drop the slop from the commit messages.

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[ychampion]

Ok looking

[ychampion]

Done. I rewrote both commit messages as concise subjects and verified that the resulting tree is unchanged.

[djc]

Done. I rewrote both commit messages as concise subjects and verified that the resulting tree is unchanged.

Did you miss the earlier feedback from @rami3l?

[ychampion]

Done. I rewrote both commit messages as concise subjects and verified that the resulting tree is unchanged.

Did you miss the earlier feedback from @rami3l?

Yeah maybe, could you reference it?

[djc]

Done. I rewrote both commit messages as concise subjects and verified that the resulting tree is unchanged.

Did you miss the earlier feedback from @rami3l?

Yeah maybe, could you reference it?

I meant the commit you added, but the new commit should be squashed into the first one.

[ychampion]

Done. I folded the follow-up into the first commit, so the branch is back to two commits.

[rami3l]

Thank you :)

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