Update MaxText Package Tests workflow run conditions

[SurbhiJainUSC]

Description

This PR updates the conditions for running tests and notebooks in the MaxText package workflow.

• The job previously named doc_only_check has been renamed to analyze_code_changes to better reflect its expanded role in orchestrating tests.
• A new check has been added to detect changes within the src/dependencies/ and .github/workflows directory. If changes are detected in these directories, the workflow now automatically enables both unit tests and Jupyter notebooks.
• A new check has been added to detect changes within the src/maxtext/trainers/post_train/ directory. If changes are detected, the workflow now automatically enables Jupyter notebooks.
• Downstream jobs, such as build_and_upload_maxtext_package, tpu-tests, and maxtext_jupyter_notebooks, have been updated to depend on the output of the new analyze_code_changes job.

Tests

CI tests

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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