fix: preserve repeated embedded marker prefixes

[vycdev2]

Summary

• Parse embedded todo match metadata in one shared helper used by both live-document and ag-provider paths.
• Use each regex match position for the source-code prefix, so repeated identical markers on one line no longer point at the first marker.
• Add regression coverage and document the user-visible fix.

Verification

• npm test — passed: 135 tests.
• npm run compile — passed: webpack development build completed.
• npm run format:check — passed: all matched files use Prettier style.
• git diff --check upstream/develop...HEAD — passed.

Risk

• Low: the parsing fields are unchanged except for the corrected prefix position; both provider paths now share the same tested mapping.

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