fix: ignore timestamp completions in multi-backtick code

[vycdev2]

Summary

• Match opening and closing backtick runs when checking timestamp completion prefixes.
• Prevent timestamp completion inside multi-backtick inline code while preserving completion after the closed span.
• Add regression coverage and a Version 5.3.0 changelog entry.

Verification

• npm install — passed; npm reported 30 audit vulnerabilities and deprecation warnings.
• Focused: npm test -- --grep 'multi-backtick code spans' — passed (1 test).
• npm test — passed (135 tests).
• npm run compile — passed.
• npm run format:check — passed.
• git diff --check upstream/develop...HEAD — passed.

Risk

The change is limited to timestamp completion's inline-code boundary detection; no runtime dependencies or configuration changes.

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