ci: add branch check and test timeouts - #3139

Summary

Prevent branch checks from running indefinitely and make hung Rust tests identify themselves in the CI log. This follows Branch Checks run 33643869577, where all three Rust test jobs remained active for more than two hours.

The 30-minute job cap is more than twice the slowest sampled successful job, including cold-cache runs, which completed within 14 minutes 17 seconds.

Related Issue

No issue required: this is a localized CI reliability and diagnostics change.

Changes

• Add a 30-minute timeout to all eight substantive branch-check jobs
• Report nextest tests as slow after 30 seconds
• Terminate individual tests after two minutes and report them as timed out

Testing

• mise run pre-commit passes
• Branch Checks workflow parses as YAML and all substantive jobs have the expected timeout
• Nextest configuration parses as TOML with the expected slow-test settings
• git diff --check passes
• actionlint was unavailable locally; GitHub Actions will validate the workflow

Checklist

• Follows Conventional Commits
• Commit is signed off for DCO
• Architecture documentation is not applicable to this CI-only change