gpui: Avoid redundant SharedString allocations in ShapedLine::split_at (#54583)

Build split text directly from Arc<str> slices instead of allocating
intermediate Strings. Reuse the original SharedString for full-line
splits, return a static empty string for empty sides, and add a
regression test covering boundary splits.

Release Notes:

- N/A

---------

Signed-off-by: Xiaobo Liu <cppcoffee@gmail.com>

crates/gpui/src/text_system/line.rs

@@ -208,8 +208,16 @@ impl ShapedLine {
         }
 
         // Split text
-        let left_text = SharedString::new(self.text[..byte_index].to_string());
-        let right_text = SharedString::new(self.text[byte_index..].to_string());
+        let left_text = if byte_index == self.text.len() {
+            self.text.clone()
+        } else {
+            SharedString::new(&self.text[..byte_index])
+        };
+        let right_text = if byte_index == 0 {
+            self.text.clone()
+        } else {
+            SharedString::new(&self.text[byte_index..])
+        };
 
         let left_width = x_offset;
         let right_width = self.layout.width - left_width;