agent: Do not render unnecessary lines in edit file tool card (#29766)

This PR prevents any unnecessary lines from being rendered in the edit
file tool card in the case of small diffs.

I think this (hopefully) addresses the last remaining task from
https://github.com/zed-industries/zed/pull/29448.

| `main` | This PR |
| --- | --- |
| <img width="634" alt="main"
src="https://github.com/user-attachments/assets/7c06394e-957a-4d36-a484-5974687041e9"
/> | <img width="634" alt="PR"
src="https://github.com/user-attachments/assets/84206d5a-a93a-4a42-99ca-7cdebb0d91bb"
/> |

(The last empty line in the second image is an empty line present in the
file itself)

---

n the second commit I also preemtively disabled vertical overscrolling
for full mode editors which are sized by content. This is basically the
same fix as in https://github.com/zed-industries/zed/pull/28471.
Strictly speaking, this is not needed for the fix here, but I thought it
might be nice to have for the future to prevent any issues from occuring
due to overscroll.

Release Notes:

- agent: Improved rendering of small diffs for the edit file tool card.

crates/assistant_tools/src/edit_file_tool.rs

@@ -577,13 +577,10 @@ impl ToolCard for EditFileToolCard {
                 card.child(
                     v_flex()
                         .relative()
-                        .map(|editor_container| {
-                            if self.full_height_expanded {
-                                editor_container.h_full()
-                            } else {
-                                editor_container
-                                    .h(DEFAULT_COLLAPSED_LINES as f32 * editor_line_height)
-                            }
+                        .h_full()
+                        .when(!self.full_height_expanded, |editor_container| {
+                            editor_container
+                                .max_h(DEFAULT_COLLAPSED_LINES as f32 * editor_line_height)
                         })
                         .overflow_hidden()
                         .border_t_1()