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/assistant/Cargo.toml

@@ -73,6 +73,7 @@ settings.workspace = true
 similar.workspace = true
 smallvec.workspace = true
 smol.workspace = true
+strum.workspace = true
 telemetry_events.workspace = true
 terminal.workspace = true
 terminal_view.workspace = true