Improve mutation test coverage for line selection parsing

[sanjeevpenupala]

Summary

• Add 16 new test cases across checkLegacySyntax.test.ts and parseLineSelector.test.ts to kill 11 survived mutants
• Covered mutation score rises from 93.42% to 98.25%
• Remaining 4 survivors are equivalent mutants or unreachable dead code

Details

checkLegacySyntax.test.ts (8 new tests):

• Inputs without a colon (should not throw)
• Non-numeric, empty, whitespace-only, and mixed alphanumeric suffixes after colon (should not throw)
• Legacy syntax variants including single line, spaces in selector, and colon at index 1 (should throw)

parseLineSelector.test.ts (8 new tests):

• Whitespace-padded selectors and spaces around comma-separated parts
• Trailing and double commas (empty part handling)
• Old dash syntax detection with whitespace around parts
• Negative numbers with leading spaces not falsely flagged as old dash syntax

Test plan

• All 129 tests pass (vitest run)
• Full build pipeline passes (npm run build)
• Mutation score above break threshold (76.98% total, 98.25% covered)