Split TPU and GPU nightly image builds to run E2E tests in parallel:

[chiajunglien]

• Split the single build_and_push_docker_image matrix job into parallel build_and_push_tpu_docker_images and build_and_push_gpu_docker_images jobs.
• Update run_e2e_tests to depend only on the TPU builds, allowing the Airflow E2E tests to trigger immediately after TPU pre/post-training images are pushed.
• Ensure GPU CI tests run in parallel with the Airflow E2E tests, drastically reducing overall pipeline runtime.
• Correct the maxtext_sha reference to use the correct job ID build_maxtext_package.
• Update notify_failure to monitor both build jobs and the E2E tests.

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[codecov]

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✅ All modified and coverable lines are covered by tests.

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