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 owned pool routing during a cloud overflow outage so the picker no longer places or retries jobs onto a dead Blacksmith pool. Follow-up to cmux#15614 for an explicitly Blacksmith-only CI_PR_POOL_ORDER.

Bug Fixes

• While CI_CLOUD_OVERFLOW_SAVED is present (and its recorded lane matches the live MACOS_RUNNER_PR), the picker keeps normal owned placement but never emits a non-owned candidate or retry target.
• A Blacksmith-only configured order is rebuilt as the owned default for the pinned PR Xcode instead of falling back to an unmeasured direct route.

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

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[teamleaderleo]

Superseded by #15620 after rebasing the dependent change onto the current picker branch.