fix: preserve prefix in blink.cmp org directive completions

[seflue]

Summary

This PR fixes a bug where blink.cmp autocompletion incorrectly removes the #+ prefix when completing org directives. When typing #+fi and selecting #+filetype from the completion menu, the completion would incorrectly insert just filetype instead of #+filetype.

Related Issues

This PR addresses the bug described in my bug report #977 .

Changes

• Fixed the get_completions function in lua/orgmode/org/autocompletion/blink.lua to preserve the #+ prefix for org directives
• Added a test case in tests/plenary/org/autocompletion_spec.lua to verify that #+ prefixes are preserved when completing org directives
• The fix ensures that other completion types (tags, headlines, etc.) continue to work as before while only affecting org directive completions

Checklist

I confirm that I have:

• Followed the
  Conventional Commits
  specification (e.g., feat: add new feature, fix: correct bug,
  docs: update documentation).
• My PR title also follows the conventional commits specification.
• Updated relevant documentation, if necessary.
• Thoroughly tested my changes.
• Added tests (if applicable) and verified existing tests pass with
  make test.
• Checked for breaking changes and documented them, if any.

[kristijanhusak]

Works great, but lets move logic around a bit.

[kristijanhusak]

Thanks!