Added validation tests for user data endpoints.

[AndrewWHans]

Description

Related Issue

Fixes #565

Type of change

• Bug fix
• New feature / enhancement
• Documentation update
• Test addition
• Refactor

Checklist

• I have read CONTRIBUTING.md
• My branch is up to date with main
• I have run pytest -v and all tests pass
• I have not introduced duplicate issues or features
• My PR title follows the format: feat/fix/docs/test: short description
• I have added tests for new features (Level 2 and 3 issues)
• No hardcoded secrets or API keys in my code
• This PR is linked to a GSSoC 2026 issue

Screenshots (if frontend change)

Test evidence

python -m pytest tests/test_user_data.py -v

5 passed

python -m pytest

404 passed

[AndrewWHans]

Opened PR #949 adding regression tests for missing/null field validation in /user/history and /user/favorites. Verified new tests pass and full test suite passes (404 tests).