agent: Improve layout shift on previous message editor (#33055)

Follow up to https://github.com/zed-industries/zed/pull/32765.

This PR creates a slot for the `message_editor::create_editor` to allow
using different values for min and max lines. In practice, the panel's
main editor now has a minimum of 4 lines, whereas the previous message
editor has just one. This makes the layout shift when clicking on a
previous message to edit it much smaller.

Release Notes:

- agent: Improved layout shift when clicking to edit a previous sent
message.

crates/agent/src/active_thread.rs

@@ -57,6 +57,10 @@ use workspace::{CollaboratorId, Workspace};
 use zed_actions::assistant::OpenRulesLibrary;
 use zed_llm_client::CompletionIntent;
 
+const CODEBLOCK_CONTAINER_GROUP: &str = "codeblock_container";
+const EDIT_PREVIOUS_MESSAGE_MIN_LINES: usize = 1;
+const EDIT_PREVIOUS_MESSAGE_MAX_LINES: usize = 6;
+
 pub struct ActiveThread {
     context_store: Entity<ContextStore>,
     language_registry: Arc<LanguageRegistry>,
@@ -334,8 +338,6 @@ fn tool_use_markdown_style(window: &Window, cx: &mut App) -> MarkdownStyle {
     }
 }
 
-const CODEBLOCK_CONTAINER_GROUP: &str = "codeblock_container";
-
 fn render_markdown_code_block(
     message_id: MessageId,
     ix: usize,
@@ -1327,6 +1329,8 @@ impl ActiveThread {
             self.context_store.downgrade(),
             self.thread_store.downgrade(),
             self.text_thread_store.downgrade(),
+            EDIT_PREVIOUS_MESSAGE_MIN_LINES,
+            EDIT_PREVIOUS_MESSAGE_MAX_LINES,
             window,
             cx,
         );