fix(core): handle malformed Markdown heading levels

[Aming9303]

Summary

• parse heading levels defensively instead of raising on malformed user data
• preserve valid h1-h6 headings and clamp numeric levels to Markdown's h1-h6 range
• fall back to h2 for missing, empty, None, and non-heading values
• add regression coverage for valid, malformed, and out-of-range levels

Verification

• pytest tests/test_core/test_models.py -q (39 passed, 5 skipped)
• pytest tests -q (546 passed, 9 skipped, 19 deselected)
• ruff check src tests
• ruff format --check src tests

Fixes #176

[vedaant00]

Reviewed and it's correct @Aming9303: title/empty/None default to h2, h9/h0 clamp to h1-h6, and valid levels are unchanged. 7 tests pass, green, no conflicts. Merging, thanks and welcome.