assistant: Polish /workflow and steps UI (#15936)

Fixes #15923
Release Notes:

- Assistant workflow steps can now be applied and reverted directly from
within the assistant panel.

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Antonio <antonio@zed.dev>

crates/editor/src/editor.rs

@@ -9614,6 +9614,7 @@ impl Editor {
                                 }
                             }),
                             disposition: BlockDisposition::Below,
+                            priority: 0,
                         }],
                         Some(Autoscroll::fit()),
                         cx,
@@ -9877,6 +9878,7 @@ impl Editor {
                             height: message_height,
                             render: diagnostic_block_renderer(diagnostic, None, true, true),
                             disposition: BlockDisposition::Below,
+                            priority: 0,
                         }
                     }),
                     cx,
@@ -10182,6 +10184,7 @@ impl Editor {
         if let Some(autoscroll) = autoscroll {
             self.request_autoscroll(autoscroll, cx);
         }
+        cx.notify();
         blocks
     }
 
@@ -10196,6 +10199,7 @@ impl Editor {
         if let Some(autoscroll) = autoscroll {
             self.request_autoscroll(autoscroll, cx);
         }
+        cx.notify();
     }
 
     pub fn replace_blocks(
@@ -10208,9 +10212,8 @@ impl Editor {
             .update(cx, |display_map, _cx| display_map.replace_blocks(renderers));
         if let Some(autoscroll) = autoscroll {
             self.request_autoscroll(autoscroll, cx);
-        } else {
-            cx.notify();
         }
+        cx.notify();
     }
 
     pub fn remove_blocks(
@@ -10225,6 +10228,7 @@ impl Editor {
         if let Some(autoscroll) = autoscroll {
             self.request_autoscroll(autoscroll, cx);
         }
+        cx.notify();
     }
 
     pub fn row_for_block(