refactor(github): convert github utils to TaskFlow @task and chain

[RexBearIU]

Description

Refactored xlml/utils/github.py and dags/multipod/maxtext_e2e_tests.py to follow standard Airflow 2.x TaskFlow conventions:

• Converted validate_git_trigger and fire_github_callback to TaskFlow tasks using @task.
• Replaced **context with get_current_context().
• Replaced >> bitshift operators with chain().
• Standardized DAG parameter names to commit_sha and github_token (with backward-compatible fallbacks).
• Added unit tests in xlml/utils/github_test.py.

Tests

1. Pre-commit & Unit Tests:

pre-commit run --all-files
python3 -m unittest xlml/utils/github_test.py

2. Cloud Composer Integration Tests:
   Uploaded DAG and utility files to test environment jackyf-test and verified task structure and import errors:

gcloud storage cp dags/multipod/maxtext_e2e_tests.py gs://us-east1-jackyf-test-3fc56b08-bucket/dags/multipod/maxtext_e2e_tests.py
gcloud storage cp xlml/utils/github.py gs://us-east1-jackyf-test-3fc56b08-bucket/dags/xlml/utils/github.py
gcloud composer environments run jackyf-test --location us-east1 --project cloud-ml-auto-solutions dags list-import-errors
gcloud composer environments run jackyf-test --location us-east1 --project cloud-ml-auto-solutions tasks list -- maxtext_e2e_tests

Output from Composer:

• dags list-import-errors: No import errors for maxtext_e2e_tests.
• tasks list: Successfully listed all tasks:
  • <Task(_TaskDecorator): fire_github_callback_post_training>
  • <Task(_TaskDecorator): fire_github_callback_pre_training>
  • <Task(TriggerDagRunOperator): trigger_tpu_post_training>
  • <Task(TriggerDagRunOperator): trigger_tpu_pre_training>
  • <Task(_TaskDecorator): validate_git_trigger>

Checklist

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run one-shot tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.