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/editor/src/element.rs

@@ -6874,7 +6874,12 @@ impl Element for EditorElement {
                     // The max scroll position for the top of the window
                     let max_scroll_top = if matches!(
                         snapshot.mode,
-                        EditorMode::AutoHeight { .. } | EditorMode::SingleLine { .. }
+                        EditorMode::SingleLine { .. }
+                            | EditorMode::AutoHeight { .. }
+                            | EditorMode::Full {
+                                sized_by_content: true,
+                                ..
+                            }
                     ) {
                         (max_row - height_in_lines + 1.).max(0.)
                     } else {