pane: Fix double or invisible borders in tab bar (#29061)

Invisible borders:
<img width="349" alt="Screenshot 2025-04-18 at 3 59 03 PM"
src="https://github.com/user-attachments/assets/a3a43885-ce87-4fcf-864a-d730fea1551e"
/>
<img width="547" alt="Screenshot 2025-04-18 at 8 23 15 PM"
src="https://github.com/user-attachments/assets/1f8669a8-f893-4c58-ba30-025be1bc733f"
/>

Double borders:
<img width="295" alt="Screenshot 2025-04-18 at 3 56 48 PM"
src="https://github.com/user-attachments/assets/7b4ae42d-c7fd-478c-97ce-10abefe4a482"
/>

Release Notes:

- N/A

crates/workspace/src/pane.rs

@@ -2661,12 +2661,12 @@ impl Pane {
                     tab_bar
                 }
             })
-            .children(pinned_tabs.len().ne(&0).then(|| {
-                h_flex()
-                    .children(pinned_tabs)
-                    .border_r_2()
-                    .border_color(cx.theme().colors().border)
-            }))
+            .children(
+                pinned_tabs
+                    .len()
+                    .ne(&0)
+                    .then(|| h_flex().children(pinned_tabs)),
+            )
             .child(
                 h_flex()
                     .id("unpinned tabs")