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/editor/src/display_map.rs

@@ -105,7 +105,7 @@ pub struct DisplayMap {
     inlay_highlights: InlayHighlights,
     /// A container for explicitly foldable ranges, which supersede indentation based fold range suggestions.
     crease_map: CreaseMap,
-    fold_placeholder: FoldPlaceholder,
+    pub(crate) fold_placeholder: FoldPlaceholder,
     pub clip_at_line_ends: bool,
     pub(crate) masked: bool,
 }