fix: use isupper/islower for case validators

[Chessing234]

Summary

• text == text.upper()/lower() accepted empty and digit-only strings.
• Use str.isupper / str.islower like IFEvalG.

Test plan

• TestValidateUpperLower

GPU_TESTS=bypass
CHANGELOG=

[gemini-code-assist]

Code Review

This pull request updates the validate_uppercase and validate_lowercase functions in open_instruct/if_functions.py to use Python's built-in str.isupper() and str.islower() methods instead of comparing the string to its upper/lower-cased version. This ensures that empty strings and strings containing only non-letter characters correctly fail validation. Additionally, corresponding unit tests have been added to open_instruct/test_if_functions.py to verify these behaviors, and the CHANGELOG.md has been updated. There are no review comments, so I have no feedback to provide.

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