test: add edge-case coverage for validation helper functions

[jyotish6699]

Summary

This PR adds additional edge-case coverage for validation helper functions in utils/validation.py.

Added Test Cases

• Verify custom error_invalid handling in get_choice()
• Verify empty input handling in get_yes_no()
• Verify empty input handling in get_int_list()
• Verify empty input handling in get_float_list()
• Verify whitespace-only input handling in get_non_empty_string()

Motivation

These scenarios exercise existing validation branches that were previously not covered by automated tests. Improving coverage helps prevent regressions and ensures consistent user input handling across mini-projects that depend on these helper utilities.

Verification

• Added new edge-case tests in tests/test_validation.py
• All existing tests pass successfully
• Full test suite passes locally (236 passed)

Closes #1296

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[steam-bell-92]

@jyotish6699 i've deleted all test files so this one also not merging