Immediate edit step resolution (#16447)

## Todo

* [x] Parse and present new XML output
* [x] Resolve new edits to buffers and anchor ranges
* [x] Surface resolution errors
* [x] Steps fail to resolve because language hasn't loaded yet
* [x] Treat empty `<symbol>` tag as None
* [x] duplicate assists when editing steps
* [x] step footer blocks can appear *below* the following message header
block

## Release Notes:

- N/A

---------

Co-authored-by: Mikayla <mikayla@zed.dev>
Co-authored-by: Peter <peter@zed.dev>
Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/language/src/buffer.rs

@@ -21,7 +21,7 @@ use async_watch as watch;
 pub use clock::ReplicaId;
 use futures::channel::oneshot;
 use gpui::{
-    AnyElement, AppContext, EventEmitter, HighlightStyle, ModelContext, Task, TaskLabel,
+    AnyElement, AppContext, EventEmitter, HighlightStyle, ModelContext, Pixels, Task, TaskLabel,
     WindowContext,
 };
 use lsp::LanguageServerId;
@@ -40,7 +40,7 @@ use std::{
     future::Future,
     iter::{self, Iterator, Peekable},
     mem,
-    ops::{Deref, Range},
+    ops::{Deref, DerefMut, Range},
     path::{Path, PathBuf},
     str,
     sync::{Arc, LazyLock},
@@ -486,11 +486,16 @@ pub struct Chunk<'a> {
 #[derive(Clone)]
 pub struct ChunkRenderer {
     /// creates a custom element to represent this chunk.
-    pub render: Arc<dyn Send + Sync + Fn(&mut WindowContext) -> AnyElement>,
+    pub render: Arc<dyn Send + Sync + Fn(&mut ChunkRendererContext) -> AnyElement>,
     /// If true, the element is constrained to the shaped width of the text.
     pub constrain_width: bool,
 }
 
+pub struct ChunkRendererContext<'a, 'b> {
+    pub context: &'a mut WindowContext<'b>,
+    pub max_width: Pixels,
+}
+
 impl fmt::Debug for ChunkRenderer {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.debug_struct("ChunkRenderer")
@@ -499,6 +504,20 @@ impl fmt::Debug for ChunkRenderer {
     }
 }
 
+impl<'a, 'b> Deref for ChunkRendererContext<'a, 'b> {
+    type Target = WindowContext<'b>;
+
+    fn deref(&self) -> &Self::Target {
+        self.context
+    }
+}
+
+impl<'a, 'b> DerefMut for ChunkRendererContext<'a, 'b> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        self.context
+    }
+}
+
 /// A set of edits to a given version of a buffer, computed asynchronously.
 #[derive(Debug)]
 pub struct Diff {