feat: Add unit tests for line_utils functions

[gittihub-jpg]

Closes #543

Description

Adds comprehensive unit tests for line_utils to prevent incorrect position mapping during analysis.

• Covers splitting, indexing, and line number mapping functions in backend/app/services/line_utils.py.
• Uses pytest parametrization to validate boundary conditions, large inputs, and malformed data.

Special Notes

• backend/tests/test_line_utils.py: Parametrized test suite for split, index, and mapping utilities.
• backend/app/services/line_utils.py: No functional changes; only test coverage added.

Related Issue

Fixes #543

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
• Backwards compatibility maintained

Screenshots (if applicable)