Remove block_mouse_down in favor of stop_mouse_events_except_scroll (#30401)

This method was added in #20649 to be an alternative of `occlude` which
allows scroll events. It seems a bit arbitrary to only stop left mouse
downs, so this seems like it's probably an improvement.

Release Notes:

- N/A

crates/agent/src/inline_assistant.rs

@@ -1445,7 +1445,7 @@ impl InlineAssistant {
                     style: BlockStyle::Flex,
                     render: Arc::new(move |cx| {
                         div()
-                            .block_mouse_down()
+                            .block_mouse_except_scroll()
                             .bg(cx.theme().status().deleted_background)
                             .size_full()
                             .h(height as f32 * cx.window.line_height())