fix-I/O-robustness and Improve CI test selection script robustness

[ANSHIKATYAGI30]

Description

This PR improves the robustness and maintainability of the CI test selection script by addressing issues related to exception handling, change-detection visibility, and file I/O consistency.

Changes made-:

1. Replaced broad except Exception handling in get_changed_files() with except subprocess.CalledProcessError to avoid masking unexpected errors.
2. Added a warning message when no changed files are detected before falling back to running the full test suite, making change-detection issues more visible during CI runs.
3. Added encoding="utf-8" when writing to GITHUB_OUTPUT for consistent file handling across platforms.

Related Issues

Closes #774

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refractor

How Has This Been Tested?

1. Ran the script locally with various file-change scenarios.
2. Confirmed that the backend/frontend test selection behavior remains unchanged.

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.

[ANSHIKATYAGI30]

@utksh1 Please review the PR whenever convenient.
Thanks!