gpui: Fix text ellipsis appearing even the flex element has space (#17149)

Release Notes:

- N/A

There was a calculation bug before. When we added `text_ellipsis` to the
flex element, it would always show ellipsis no matter how long it was.

Actually we can't use `flex` and `text_ellipsis` at same time, the CSS
also not support this. But this bug will let user confuse.

### Before

<img width="731" alt="image"
src="https://github.com/user-attachments/assets/b7d60017-6785-45f5-8b40-dd5efa154a1e">

### After

<img width="521" alt="image"
src="https://github.com/user-attachments/assets/a3117793-284e-48d4-8c15-059fe61abe60">

crates/gpui/examples/text_wrapper.rs

@@ -13,6 +13,36 @@ impl Render for HelloWorld {
             .p_2()
             .gap_2()
             .bg(gpui::white())
+            .child(
+                div()
+                    .flex()
+                    .flex_row()
+                    .gap_2()
+                    .child(
+                        div()
+                            .flex()
+                            .border_1()
+                            .border_color(gpui::red())
+                            .text_ellipsis()
+                            .child("longer text in flex 1"),
+                    )
+                    .child(
+                        div()
+                            .flex()
+                            .border_1()
+                            .border_color(gpui::red())
+                            .text_ellipsis()
+                            .child("short flex"),
+                    )
+                    .child(
+                        div()
+                            .overflow_hidden()
+                            .border_1()
+                            .border_color(gpui::red())
+                            .text_ellipsis()
+                            .child("A short text in normal div"),
+                    ),
+            )
             .child(
                 div()
                     .text_xl()