agent: Update thread label to use plural form (#27971)

Update thread label to match the other contexts.

|Before|After|
|--|--|

|![image](https://github.com/user-attachments/assets/6e02808e-50d7-480f-a9ca-251e9519a71d)|![image](https://github.com/user-attachments/assets/174aad84-9e55-4531-bb4a-1a1adaa46418)|

Release Notes:

- N/A

crates/agent/src/context_picker.rs

@@ -76,7 +76,7 @@ impl ContextPickerMode {
             Self::File => "Files & Directories",
             Self::Symbol => "Symbols",
             Self::Fetch => "Fetch",
-            Self::Thread => "Thread",
+            Self::Thread => "Threads",
         }
     }