Introduce dynamic tab titles for unsaved files based on buffer content (#32353)

https://github.com/user-attachments/assets/0bb08784-251c-4221-890a-2d6b3fb94e0f

For new, unsaved files:

- If a buffer has no content, or contains only whitespace, use
`untitled`
- If a buffer has content, take the first 40 chars of the first line

| Sublime | VS Code | Zed |
|---------|---------|-----|
| <img width="227" alt="SCR-20250608-ouux"
src="https://github.com/user-attachments/assets/d02b1e50-5775-4252-86e6-6c9d3f6c72fb"
/> | <img width="230" alt="SCR-20250608-ousn"
src="https://github.com/user-attachments/assets/7c9c016b-642f-4a80-9bc1-8c9bdc7bbd32"
/> | <img width="242" alt="SCR-20250608-ovbg"
src="https://github.com/user-attachments/assets/c7f4be5c-5bba-4a2a-b477-1392ca938cd5"
/> |

Note that this implementation also trims all leading whitespace, so that
if the buffer has any non-whitespace content, we use it. VS Code and
Sublime do not do this.

| Sublime | VS Code | Zed |
|---------|---------|-----|
| <img width="233" alt="SCR-20250608-oviq"
src="https://github.com/user-attachments/assets/ccffecc6-0f46-4d1b-8739-740240bc067b"
/> | <img width="198" alt="SCR-20250608-ovkq"
src="https://github.com/user-attachments/assets/35c20149-f898-417b-aff3-dda22b8cc1f3"
/> | <img width="233" alt="SCR-20250608-ovns"
src="https://github.com/user-attachments/assets/2509e8f6-254b-4fcb-a0ea-e18e95bb685b"
/> |

Release Notes:

- Introduced dynamic tab titles for unsaved files based on buffer
content

crates/multi_buffer/src/multi_buffer_tests.rs

@@ -3651,3 +3651,59 @@ fn assert_line_indents(snapshot: &MultiBufferSnapshot) {
         "reversed_line_indents({max_row})"
     );
 }
+
+#[gpui::test]
+fn test_new_empty_buffer_uses_untitled_title(cx: &mut App) {
+    let buffer = cx.new(|cx| Buffer::local("", cx));
+    let multibuffer = cx.new(|cx| MultiBuffer::singleton(buffer.clone(), cx));
+
+    assert_eq!(multibuffer.read(cx).title(cx), "untitled");
+}
+
+#[gpui::test]
+fn test_new_empty_buffer_uses_untitled_title_when_only_contains_whitespace(cx: &mut App) {
+    let buffer = cx.new(|cx| Buffer::local("\n ", cx));
+    let multibuffer = cx.new(|cx| MultiBuffer::singleton(buffer.clone(), cx));
+
+    assert_eq!(multibuffer.read(cx).title(cx), "untitled");
+}
+
+#[gpui::test]
+fn test_new_empty_buffer_takes_first_line_for_title(cx: &mut App) {
+    let buffer = cx.new(|cx| Buffer::local("Hello World\nSecond line", cx));
+    let multibuffer = cx.new(|cx| MultiBuffer::singleton(buffer.clone(), cx));
+
+    assert_eq!(multibuffer.read(cx).title(cx), "Hello World");
+}
+
+#[gpui::test]
+fn test_new_empty_buffer_takes_trimmed_first_line_for_title(cx: &mut App) {
+    let buffer = cx.new(|cx| Buffer::local("\nHello, World ", cx));
+    let multibuffer = cx.new(|cx| MultiBuffer::singleton(buffer.clone(), cx));
+
+    assert_eq!(multibuffer.read(cx).title(cx), "Hello, World");
+}
+
+#[gpui::test]
+fn test_new_empty_buffer_uses_truncated_first_line_for_title(cx: &mut App) {
+    let title_after = ["a", "b", "c", "d"]
+        .map(|letter| letter.repeat(10))
+        .join("");
+    let title = format!("{}{}", title_after, "e".repeat(10));
+    let buffer = cx.new(|cx| Buffer::local(title, cx));
+    let multibuffer = cx.new(|cx| MultiBuffer::singleton(buffer.clone(), cx));
+
+    assert_eq!(multibuffer.read(cx).title(cx), title_after);
+}
+
+#[gpui::test]
+fn test_new_empty_buffers_title_can_be_set(cx: &mut App) {
+    let buffer = cx.new(|cx| Buffer::local("Hello World", cx));
+    let multibuffer = cx.new(|cx| MultiBuffer::singleton(buffer.clone(), cx));
+    assert_eq!(multibuffer.read(cx).title(cx), "Hello World");
+
+    multibuffer.update(cx, |multibuffer, cx| {
+        multibuffer.set_title("Hey".into(), cx)
+    });
+    assert_eq!(multibuffer.read(cx).title(cx), "Hey");
+}