agent: Adjust codeblock design across edit file tool call card and Markdown (#30931)

This PR makes the edit tool call codeblock cards expanded by default, to
be consistent with https://github.com/zed-industries/zed/pull/30806.
Also, I am removing the collapsing behavior of Markdown codeblocks where
we'd add a gradient while capping the container's height based on an
arbitrary number of lines. Figured if they're all now initially
expanded, we could simplify how the design/code operates here
altogether.

Open for feedback, as I can see an argument where the previous Markdown
codeblock design of "collapsed but not fully; it shows a preview" should
stay as it is useful.

Release Notes:

- N/A

crates/assistant_tools/src/edit_file_tool.rs

@@ -400,7 +400,7 @@ impl EditFileToolCard {
             diff_task: None,
             preview_expanded: true,
             error_expanded: None,
-            full_height_expanded: false,
+            full_height_expanded: true,
             total_lines: None,
         }
     }