Fix opening excerpt when the cursor is at the end of a multi-buffer (#2522)

Fixes
https://linear.app/zed-industries/issue/Z-987/option-enter-doesnt-open-buffer-from-multibuffer-if-on-the-last-line

This was caused by seeking with a right bias, which in the case of the
last excerpt in the buffer would seek past the end of the buffer. This
commit changes the behavior to move the cursor back to the previous
excerpt if we overshoot, to ensure we always land on an excerpt.

Release Notes:

- Fixed a bug that prevented opening excerpts when the cursor was at the
end of a multi-buffer.