Fix empty labels in Stackdriver log IO for Airflow 3 Supervisor

[23tae]

This PR adds fallback logic to StackdriverRemoteLogIO to extract task labels directly from the structlog context dictionary when record.task_instance is missing, addressing the empty labels issue in the Airflow 3 Supervisor.

Description

This PR addresses Bug 1 described in #68240.

In the Airflow 3 Task SDK Supervisor environment, the task_instance object may be stripped from log records. Without it, the Stackdriver handler failed to attach identifying labels, resulting in untraceable "empty label" logs in Google Cloud.

As suggested by @ashb in the issue, rather than manually parsing log file paths, this PR leverages the structlog context variables. The Task SDK inherently binds task details to the structlog context, making them naturally available in the event dictionary.

Key changes

• Fallback Label Extraction: Added fallback logic in StackdriverRemoteLogIO.processors.proc. If record.task_instance is missing, it safely extracts dag_id, task_id, run_id, try_number, and map_index directly from the event dictionary and applies them as labels.

Verification Results

I have verified the changes using prek and breeze.

• Static Checks (Prek): Passed
• Unit Tests (Breeze): Passed

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related: #68240

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

• Yes

Generated-by: Antigravity following the guidelines

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