fix(vmm): bound automatic restart retries

[kvinwang]

Problem

VMM automatically restarted a repeatedly failing VM without a retry bound. A permanent boot error caused an infinite restart loop, resource churn, and noisy logs.

Root cause and fix

Track automatic restart attempts and stop retrying after the configured bound while reporting the terminal failure.

Implementation

The branch records the following focused implementation work:

• fix(vmm): bound automatic restart retries
• fix(vmm): validate automatic restart timing
• test(vmm): exercise automatic restart policy matrix
• test(vmm): keep restart coverage focused

Changed paths:

• dstack/vmm/src/app.rs
• dstack/vmm/src/config.rs
• dstack/vmm/src/main.rs
• dstack/vmm/vmm.toml

Scope

This PR addresses one logical vmm finding. It intentionally excludes the acceptance-test infrastructure from #841 and unrelated product fixes from #840.

Dependency and merge order

This PR is based directly on master and does not require another split product PR to merge first.

Verification

• git diff --check origin/master..origin/codex/fix-vmm-restart-policy: passed.
• The declared base was verified as an ancestor of the PR head.
• Focused compile/check verification was run for the changed component where applicable; non-Rust packaging or configuration changes were reviewed against their exact branch delta.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.