fix: Protect codersdk.Client SessionToken so it can be updated (#4965)

This feature is used by the coder agent to exchange a new token. By
protecting the SessionToken via mutex we ensure there are no data races
when accessing it.

loadtest/agentconn/run_test.go

@@ -254,7 +254,7 @@ func setupRunnerTest(t *testing.T) (client *codersdk.Client, agentID uuid.UUID)
 	coderdtest.AwaitWorkspaceBuildJob(t, client, workspace.LatestBuild.ID)
 
 	agentClient := codersdk.New(client.URL)
-	agentClient.SessionToken = authToken
+	agentClient.SetSessionToken(authToken)
 	agentCloser := agent.New(agent.Options{
 		Client: agentClient,
 		Logger: slogtest.Make(t, nil).Named("agent"),

loadtest/workspacebuild/run_test.go

@@ -129,7 +129,7 @@ func Test_Runner(t *testing.T) {
 				i := i + 1
 
 				agentClient := codersdk.New(client.URL)
-				agentClient.SessionToken = authToken
+				agentClient.SetSessionToken(authToken)
 				agentCloser := agent.New(agent.Options{
 					Client: agentClient,
 					Logger: slogtest.Make(t, nil).