Closes #56973, Follow-up for
https://github.com/zed-industries/zed/pull/56976

When a completion's `additionalTextEdits` contained a zero-width
insertion touching the edge of the primary edit, the overlap check
treated it as overlapping and silently skipped it. This broke
rust-analyzer's ref-match completions (`&foo`), which deliver the `&` as
a zero-width additional edit at the primary edit's start (#56973). The
same root cause previously broke auto-imports at the very start of a
file (#26136), which was worked around in #37746 with a special case for
edits at position (0, 0).

This PR replaces that special case with a general rule: a zero-width
additional edit only overlaps the primary edit when it falls strictly
inside it. i.e. touching the boundary is fine.

This one check handles both the file-start auto-import case and the `&`
ref-match case, so the (0, 0) workaround from #37746 is removed.
Non-insertion edits still go through the original overlap check from
#1871.

Added three regression tests:
- the file-start auto-import (#26136)
-  the `&` insertion at the primary edit's start (#56973)
- overlapping edits being skipped while non-overlapping ones apply
(general skip case)

Release Notes:

- Fixed rust-analyzer completions like `&some_var` inserting only the
variable name and dropping the leading `&` when accepted.