Add API endpoint and UI for deadline callback log visibility

[seanghaeli]

Summary

Make deadline callback logs visible in the Airflow UI. When a deadline is missed and a callback fires, users can now view the callback's execution logs directly from the web interface.

Changes

Backend:

• New CallbackLogReader utility (airflow-core/src/airflow/utils/log/callback_log_reader.py) — reads callback logs from remote or local storage using the same RemoteLogIO infrastructure as task logs
• New API endpoint GET /ui/dags/{dag_id}/dagRuns/{dag_run_id}/callbacks/{callback_id}/logs on the deadlines UI router
• Updated DeadlineResponse datamodel — added callback_id and callback_state fields

Frontend:

• CallbackLogViewer.tsx component — fetches and displays callback execution logs in a modal dialog
• Simplified deadline status display: inline "Missed" badge + "Callback Logs" button + expected/actual times (no success badge, no "Finished late" text, no alert name)

Motivation

Kaxil requested this: "Some way to know if a deadline didn't fire and it failed: something on UI would be great, like Task log. If deadlines are visible, it would be even better experience than previous SLAs."

Dependencies

• Builds on Add remote log upload support for callback subprocesses #66379 (remote log upload for callbacks) which uploads the logs to storage
• The log path format is executor_callbacks/{dag_id}/{run_id}/{callback_id}

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Opus 4.6)

Generated-by: Claude Code (Opus 4.6) following the guidelines

End-to-End Verification

Verified in breeze with full Airflow stack (scheduler + dag-processor + triggerer + API server + LocalExecutor). A test DAG with a SyncCallback deadline was parsed, unpaused, scheduled, and the deadline missed naturally. The callback executed through supervise_callback() and produced real structured logs.

Dag Run Page — Deadline Status with Callback

Callback Logs Modal

Verified:

• 6/6 API unit tests pass (test_callback_logs.py)
• 12/12 log reader unit tests pass (test_callback_log_reader.py)
• Path traversal protection via regex validation + os.path.commonpath containment check
• Stress tested with 10K-100K line logs: 10K lines renders in 0.23s, acceptable for realistic callback output

[vincbeck]

I like the interface but I would like to have @bbovenzi opinion on this one

[seanghaeli]

@ashb updated to use TaskLogContent

[SameerMesiah97]

I have not inspected the diff itself, but I saw your dev list post requesting feedback on potential UI additions.

Overall, I think you've covered most of what users would reasonably expect to see for DeadlineAlerts in the UI. One possible enhancement would be surfacing a few additional pieces of metadata:

1. Configured interval value
2. Interval source (timedelta vs VariableInterval)
3. Callback type (for example, SlackWebhookNotifier)

Users can always inspect the DAG definition for this information, but exposing it directly in the UI would make deadline investgation more self-service. For e.g., if critical SLA is missed and a DeadlineAlert fires, an operator or stakeholder could immediately understand what deadline was configured and what notification mechanism was invoked without needing to inspect the DAG code.