[ci] Add /bkbot rerun-failure command for job-level failure rerun

[dao-jun]

Motivation

The existing /bkbot rerun command reruns entire workflows when they fail, which wastes CI resources by re-running jobs that already passed. There was no way to selectively rerun only the failed jobs while keeping successful ones intact. A job-level rerun command allows faster feedback and more efficient use of CI runners.

Changes

Added a new /bkbot rerun-failure command to the existing ci-bkbot.yaml workflow:

• Header comments & script comments: Updated to document the new command
• Supported commands array: Added 'rerun-failure' to the validation list
• New Command 3 handler: Iterates all latest workflow runs for the PR, lists all jobs, filters by failure conclusions (failure/timed_out/cancelled/skipped), and reruns each matching job individually via reRunJobForWorkflowRun
• Renumbered: Existing stop/cancel command renumbered from Command 3 to Command 4

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