editor: Fix panic when `editor::SelectLargerSyntaxNode` overflows excerpt in multi buffer (#25585)

Closes #25513

This PR handles case when `editor::SelectLargerSyntaxNode` expands
across excerpt boundaries and eventually crashes in multi buffer.

Release Notes:

- Fixed panic caused when `editor::SelectLargerSyntaxNode` is called
repetedly in multi buffer.

Co-authored-by: Ben Kunkle <ben.kunkle@gmail.com>