debugger beta: Fix dap_schema for DAP extensions (#31173)

We now actually call dap_schema provided by extensions instead of
defaulting to a null `serde_json::Value`. We still need to update the
Json LSP whenever a new dap is installed.

Release Notes:

- N/A

crates/debug_adapter_extension/src/extension_dap_adapter.rs

@@ -61,8 +61,8 @@ impl DebugAdapter for ExtensionDapAdapter {
         self.debug_adapter_name.as_ref().into()
     }
 
-    fn dap_schema(&self) -> serde_json::Value {
-        serde_json::Value::Null
+    async fn dap_schema(&self) -> serde_json::Value {
+        self.extension.get_dap_schema().await.unwrap_or_default()
     }
 
     async fn get_binary(