Run newly added tests in PRs on TPU7X runners

[darisoy]

Description

This PR enables running newly added or modified tests on TPU v7x runners (tpu7x-unit, tpu7x-integration, tpu7x-post-training-unit) during Pull Request checks (github.event_name == 'pull_request'), in addition to nightly schedules and manual workflow triggers.

Specifically, this PR:

• Updates .github/workflows/ci_pipeline.yml (tpu7x-tests) to execute on Pull Request events when package build dependencies succeed.
• Passes flavor through .github/workflows/run_tests_coordinator.yml into .github/workflows/run_tests_against_package.yml.
• Dynamically appends and newly_added to FINAL_PYTEST_MARKER for non-scheduled TPU7X test runs (INPUTS_FLAVOR =~ ^tpu7x-).
• Traps PyTest exit code 5 (No tests collected) so pull requests that do not add or modify tests complete cleanly (SUCCESS) in ~45 seconds without failing the CI pipeline.

BUGS: b/530220873
FIXES: b/530220873

Tests

Verified end-to-end on physical TPU v7x runners (tpu7x-8) across 3 scenarios:

1. Manual Trigger (workflow_dispatch) with 0 Added Tests:
   • Run URL: https://github.com/AI-Hypercomputer/maxtext/actions/runs/29053738050
   • Expected & Verified Behavior: PyTest scans all ~2,600 tests against marker ... and newly_added, matches 0 items (0 selected / 2610 deselected), and returns exit code 5. Our exit code 5 trap catches this, logs "No tests collected matching marker '... and newly_added'. Passing cleanly.", and marks all TPU7X jobs green (SUCCESS) in ~48 seconds.
2. Pull Request Trigger (pull_request) with 0 Added Tests:
   • Run URL: https://github.com/AI-Hypercomputer/maxtext/actions/runs/29109765191
   • Expected & Verified Behavior: Confirmed that native Pull Request events (PR #4422) automatically trigger all three TPU7X jobs (tpu7x-unit, tpu7x-integration, tpu7x-post-training-unit). Because 0 test functions were touched, PyTest fast-paths (0 selected, exit code 5) and completes green (SUCCESS) in ~48 seconds without reserving long TPU runtimes.
3. Selective Execution (pull_request) with 1 Added Test:
   • Run URL: https://github.com/AI-Hypercomputer/maxtext/actions/runs/29113377305
   • Expected & Verified Behavior: Confirmed on a temporary test branch adding a newly defined test (test_dummy_tpu7x_newly_added). AST diff detection (get_changed_tests()) isolated the new function and PyTest executed strictly that 1 test on tpu7x-8 hardware (1 selected / 2611 deselected), completing SUCCESS across tpu7x-unit (job 86431286035), tpu7x-integration (job 86431280394), and tpu7x-post-training-unit (job 86431280253).

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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