Send guests DiskBasedDiagnosticsFinished messages when they join a project

Co-authored-by: Antonio Scandurra <antonio@zed.dev>

crates/collab/src/rpc.rs

@@ -1012,6 +1012,21 @@ impl Server {
             }
         }
 
+        for language_server in &project.language_servers {
+            self.peer.send(
+                request.sender_id,
+                proto::UpdateLanguageServer {
+                    project_id: project_id.to_proto(),
+                    language_server_id: language_server.id,
+                    variant: Some(
+                        proto::update_language_server::Variant::DiskBasedDiagnosticsUpdated(
+                            proto::LspDiskBasedDiagnosticsUpdated {},
+                        ),
+                    ),
+                },
+            )?;
+        }
+
         Ok(())
     }