assistant edit tool: Use buffer search and replace in background (#26679)

Instead of getting the whole text from the buffer, replacing with
`String::replace`, and getting a whole diff, we'll now use `SearchQuery`
to get a range, diff only that range, and apply it (all in the
background).

When we match zero strings, we'll record a "bad search", keep going and
report it to the model at the end.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>

crates/language/src/buffer.rs

@@ -526,8 +526,8 @@ impl DerefMut for ChunkRendererContext<'_, '_> {
 /// A set of edits to a given version of a buffer, computed asynchronously.
 #[derive(Debug)]
 pub struct Diff {
-    pub(crate) base_version: clock::Global,
-    line_ending: LineEnding,
+    pub base_version: clock::Global,
+    pub line_ending: LineEnding,
     pub edits: Vec<(Range<usize>, Arc<str>)>,
 }