Fix invalid tree view task nesting across markdown structure

[scarbajali]

Description

Fixed incorrect task hierarchy detection for Obsidian markdown tasks.

The parser no longer keeps treating later indented tasks as children when a non-task bullet, intervening text, or a heading appears between them. In addition to the parser fix, the tree view now validates parent-child relationships before rendering them, so stale or invalid metadata.parent links do not incorrectly nest tasks across headings or unrelated structure.

Added regression coverage for:

• tasks separated by plain bullets
• tasks separated by intervening text
• tasks crossing heading boundaries
• tree-view validation of invalid parent links

Type of Change

• Bug fix
• New feature
• Documentation update
• Code refactoring

Checklist

• I have read and agree to the CLA
• My code follows the project's style guidelines
• I have tested my changes
• I have updated the documentation

Related Issues

Closes #577

[scarbajali]

I have read the CLA Document and I hereby sign the CLA