fix: complete tag arguments inside auto-closed parentheses

[vycdev2]

Summary

• Offer known tag-argument completions when the cursor is before an editor-inserted closing parenthesis.
• Include the existing closing parenthesis in the completion replacement range, avoiding duplicates.
• Cover the prefix helper and the completion provider path; document the user-visible fix.

Verification

• Regression test on unchanged behavior: failed as expected because completion was suppressed before ).
• npm test — passed (416 tests).
• npm run compile — passed.
• npm run format:check — passed after formatting the new test; npm run typecheck — passed.
• git diff --check upstream/develop...HEAD — passed.

Risk

• Low: only tag-argument completion range when the next character is ) changes. Dependency ID completion still has separate handling.

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