keymap_editor: Fix create keybinding button clipping out of editor (#54708)

## Summary

In the keymap editor, the search input at the top used `size_full()`
while the
adjacent button row had `min_w_96()`. On narrow panes (e.g. side-by-side
splits), this caused the action buttons — including "Create keybinding"
— to
overflow and clip out of the editor.

This change lets the search input flex and shrink (`flex_1()` +
`min_w_0()`),
and makes the button row `flex_none()` so it keeps its natural width and
stays visible at any pane size.

## Before / After

<img width="2227" height="475" alt="Screenshot 2026-04-23 215511"
src="https://github.com/user-attachments/assets/f3d79922-13e4-486b-b2b0-62ffa5d5b1a2"
/>

## Test plan

- [x] Open the keymap editor (`zed: open keymap editor`)
- [x] Confirm the "Create keybinding" and other action buttons remain
visible
- [x] Confirm the search input shrinks gracefully instead of pushing
buttons off-screen

Co-authored-by: Lukas Wirth <lukas@zed.dev>

crates/keymap_editor/src/keymap_editor.rs

@@ -2019,7 +2019,8 @@ impl Render for KeymapEditor {
                                         context.add("BufferSearchBar");
                                         context
                                     })
-                                    .size_full()
+                                    .flex_1()
+                                    .min_w_0()
                                     .h_8()
                                     .pl_2()
                                     .pr_1()
@@ -2032,7 +2033,7 @@ impl Render for KeymapEditor {
                             .child(
                                 h_flex()
                                     .gap_1()
-                                    .min_w_96()
+                                    .flex_none()
                                     .items_center()
                                     .child(
                                         IconButton::new(