[yaml] - expand jinja method

[derrickaw]

1. Fixes [Task]: YAML - update expand_jinja method #35936
2. Adds support for relative paths

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[codecov]

Codecov Report

❌ Patch coverage is 92.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.73%. Comparing base (e83a1d5) to head (064f8c2).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
sdks/python/apache_beam/yaml/yaml_transform.py	90.90%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #38547   +/-   ##
=========================================
  Coverage     58.72%   58.73%           
  Complexity     7418     7418           
=========================================
  Files          1649     1649           
  Lines        212354   212415   +61     
  Branches       6330     6330           
=========================================
+ Hits         124712   124763   +51     
- Misses        83097    83107   +10     
  Partials       4545     4545           

Flag	Coverage Δ
python	79.89% <92.50%> (+<0.01%)	⬆️

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[derrickaw]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces support for Jinja search paths and comment stripping in Beam YAML templates. Key changes include updating _BeamFileIOLoader to resolve relative paths using provided search paths and implementing strip_leading_comments to handle license headers in templates. Feedback highlights a missing shutil import in the new tests, suggests using os.path.isabs() for cross-platform path validation, and recommends avoiding bare exception handling when loading files.