fix(buffers): Fix fallback for finding a buffer by filename

[kristijanhusak]

Summary

Correctly fallback to finding an org file buffer in case it is not loaded into the state.

Related Issues

Related #1121

Closes #1121

Changes

• Use bufnr to find the buffer name
• Verify if the filename of the given buffer is actually the one we are looking for
• If not, use nvim_list_bufs

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.