Improve default task failure email template for readability

[Fury0508]

Improve default task failure email template for readability

Description

Replaces the default task failure email template to improve operator experience and mobile readability.

Problem:
Current default shows raw Python object dumps (UUIDs, repr(), internal fields), making it hard to quickly identify which DAG/task failed and find logs.

Solution:

• Professional HTML template with dark theme matching Airflow UI
• Structured display: DAG, task, run ID, attempt, host, tags
• Clear exception formatting in highlighted box
• Action buttons for "View Log" and "Mark Success"
• Plain text fallback for non-HTML email clients
• Airflow 3.x compatibility (conditional rendering for optional attributes)
• Fully backward compatible (custom templates still work)

Screenshots

Testing

• ✅ Templates render correctly with mock task data
• ✅ Handles missing URLs gracefully (Airflow 3.x compatibility)
• ✅ Email delivery tested via MailHog SMTP server
• ✅ Backward compatibility verified

Files Changed

+ airflow-core/src/airflow/templates/email/task_failure_subject.txt
+ airflow-core/src/airflow/templates/email/task_failure_body.html
+ airflow-core/src/airflow/templates/email/task_failure_body.txt
M task-sdk/src/airflow/sdk/execution_time/task_runner.py

closes: #61667

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

• Yes

Claude AI was used to:

• Design the HTML email template
• Locate correct files in Airflow 3.x codebase (task_runner.py)
• Create test scripts for validation
• Set up MailHog for email testing

The contributor reviewed all code, tested the implementation, and validated against issue requirements.

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