test: add deterministic crash checkpoints for installs

[cachebag]

Under the guidance of @rami3l, this is the first unit of work toward #4400, with #4724 as a related failure mode.

Ref: toolchain pool proposal

The plan is to establish deterministic infrastructure for testing hard interruptions without relying on debugger automation. This PR adds a test-only, environment-selected checkpoint mechanism. A child rustup process signals that it has reached the selected checkpoint and blocks; the test driver then kills and reaps it before retrying the operation in a fresh process.

This was my solution as opposed to something like a a debugger automation..but if it proves to be weak we can go that route instead. Would love feedback on this part.

The initial test interrupts installation after component payloads are installed but before toolchain metadata is written, then verifies that retrying converges to a working toolchain. This PR intentionally adds test infrastructure rather than changing recovery behavior.

(Additional named interruption points can be added as needed without changing the parent-process test harness, I figured a smaller scoped PR was appropriate first)

Follow-up work, generally speaking, will be as follows:

• Add checkpoints around additional filesystem mutations, metadata writes, and download failures.
• Reproduce and characterize the interrupted states from Interrupted transaction could get rustup stuck in loop #4400 and New parallel download + install can easily leave a partially installed toolchain when downloads are interrupted #4724.
• Add staging and atomic publication for fresh toolchain installations.
• Extend the model to updates of existing toolchains and concurrent writers.
• Address reference lifetime, abandoned-object cleanup, and eventual garbage collection separately.

[djc]

This looks okay to me.

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[rami3l]

LGTM! Make sure that you have addressed all remaining concerns and I think we can merge this.

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[cachebag]

Everything should be addressed? Unless I am mistaken.

[rami3l]

Everything should be addressed? Unless I am mistaken.

@cachebag Sorry for the confusion, I was referring to the remaining open threads but since you have pushed a fix, please feel free to resolve them directly from your side :)