fix: guard text-only item ranges

[vycdev2]

Summary

• Guard Todo item line/range access when items are parsed from raw text without a VS Code TextDocument.
• Preserve line-backed item positions when no regex match range exists.
• Keep match-text extraction compatible with the repository's TypeScript test path.
• Add regression coverage and document the user-visible fix.

Verification

• The new focused regression test failed before the implementation because range access attempted to call document APIs on a null document.
• npx mocha -r ts-node/register test/item.spec.ts --exit — passed.
• npm test — passed (135 tests).
• npm run compile — passed.
• npm run format:check — passed.
• git diff --check — passed.
• Immediate pre-publication duplicate searches returned no matching open issue or PR.

Risk

• Low: document-backed items retain their existing position calculation; only raw-text and line-only fallback paths change.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.