Unit-test step gets its own cap so a slow run can't cancel the required clippy job

[gunbai-bot]

Some runs have the non-required rust_unit_tests step (#12450) failing required CI: merge_group run 36431733521 shows it.

Cause. continue_on_error covers a step failure, not a job timeout. The step had no cap of its own. A slow unit run used up the whole 60-minute clippy job cap, GitHub cancelled the job inside the non-required step, and the witnesses aggregate failed on the cancelled job.

Fix, in the model (gunbc.compiler_gate_workflow):

• compiler_gate_unit_test_step gets its own timeout_minutes, from compiler_gate_unit_test_timeout(). A step timeout ends as a step failure, which continue_on_error does cover, so the job's conclusion no longer depends on the unit step's wall time.
• compiler_gate_clippy_timeout() is now derived as the step cap plus compiler_gate_clippy_pre_unit_reserve(). The step therefore ends inside the job by construction, not because two literals happen to agree. The job cap is unchanged (50 + 10 = 60); it is not raised to absorb the step.
• Sizing reads the clippy job records of runs 36424259908 and 36434277095 (the unit step completed) and 36431733521 (the unit step ran past the job cap).

Regenerated .github/workflows/witnesses.yml with tools.generated_artifact_gate main_wet_one. The only change is timeout-minutes on the step.

Proof that an expiring step leaves clippy green: a probe PR is built from this same model with the unit cap set to 1 minute. Its run is linked below once it finishes, and the probe is closed without merging.

🤖 Generated with Claude Code

[gunbai-bot]

The proof ran. Probe #12552 is this same model with the unit cap set to 1 minute, which the derivation turns into an 11-minute job. In run 36447742903 the step's annotation reads "The action 'Run the v1 unit tests' has timed out after 1 minutes", and the clippy job concluded success, which is the result the witnesses aggregate reads. So an expiring unit step now ends as a visible step failure, and the job's conclusion no longer depends on it. The probe is closed unmerged.

— sent from silent-koi-310