ci: recover owned order during overflow failover

[teamleaderleo]

What changed

Follow up cmux#15614's outage routing fix for an explicitly Blacksmith-only CI_PR_POOL_ORDER. During an active CI_CLOUD_OVERFLOW_SAVED record, the picker now rebuilds the owned order for the pinned PR Xcode when the configured order has no owned labels. It still refuses to emit a dead Blacksmith candidate and keeps the retry route owned.

Validation

• Focused failover picker tests pass, including the Blacksmith-only order case.
• Python compilation and git diff --check pass.

This is a dependent follow-up on #15609; it contains only the small recovery correction.

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Summary by cubic

Fixes the failover path so a hand-set Blacksmith-only pool order can't turn an active cloud overflow record into a direct, unmeasured runner route. When the configured order drops to zero owned labels, the picker now rebuilds the owned order for the pinned PR Xcode and keeps the retry route owned.

• Rebuilds the order from the owned pools for the pinned Xcode when only transient (Blacksmith) labels remain.
• Adds a regression test covering the Blacksmith-only order case.

^{Written for commit 4c06d91. Summary will update on new commits.}

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