ci: preserve owned routing during Blacksmith outage

[teamleaderleo]

What changed

When CI_CLOUD_OVERFLOW_SAVED records an active Blacksmith outage, the PR picker now:

• accepts the switched MACOS_RUNNER_PR value;
• preserves owned split, root, GUI, and warm placement;
• removes Blacksmith labels from the candidate order;
• leaves retry_runner empty so retries stay on the owned route.

The owned-pool rescue watcher pauses while the record exists, avoiding a cancellation that would re-run work onto the dead Blacksmith pool. Normal picker and rescue behavior resumes when the record is removed and the runner variables are restored.

Validation

• New picker outage and wiring tests pass.
• New rescue pause and workflow wiring tests pass.
• Python compilation and YAML parsing pass.
• git diff --check passes.

This is a dependent PR for #15609 and intentionally contains only the outage integration.

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Summary by cubic

Keeps the PR picker and owned-pool rescue on owned runners during a Blacksmith outage so work doesn't re-run onto a dead pool.

• When CI_CLOUD_OVERFLOW_SAVED records an active Blacksmith outage, the picker accepts the switched MACOS_RUNNER_PR value, preserves owned split, root, GUI, and warm placement, removes Blacksmith labels from the candidate order, and leaves retry_runner empty.
• The owned-pool rescue pauses while the record exists, avoiding a cancellation that would re-run work onto Blacksmith.
• Picker and rescue behavior resumes normally once the record is removed.

Validation

• New picker, rescue pause, and workflow wiring tests pass.
• Python compilation and YAML parsing pass; git diff --check passes.

^{Written for commit 38fbea5. Summary will update on new commits.}

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