chore: use Cow instead of String for tooltips (#2838)

A QoL change to align `Tooltip` with other elements like `Label`
Release Notes:

- N/A

crates/copilot_button/src/copilot_button.rs

@@ -140,7 +140,7 @@ impl View for CopilotButton {
                 })
                 .with_tooltip::<Self>(
                     0,
-                    "GitHub Copilot".into(),
+                    "GitHub Copilot",
                     None,
                     theme.tooltip.clone(),
                     cx,