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/src/assistant.rs

@@ -362,7 +362,7 @@ fn register_slash_commands(prompt_builder: Option<Arc<PromptBuilder>>, cx: &mut
 
     if let Some(prompt_builder) = prompt_builder {
         slash_command_registry.register_command(
-            workflow_command::WorkflowSlashCommand::new(prompt_builder),
+            workflow_command::WorkflowSlashCommand::new(prompt_builder.clone()),
             true,
         );
     }