test: use non-monitoring RPC role in apptest setup (#23953)

Closes https://github.com/coder/internal/issues/1432
Closes https://github.com/coder/internal/issues/1399

The test setup in `createWorkspaceWithApps` opens a short-lived RPC
connection to fetch the agent manifest before starting the real agent.
This connection used `ConnectRPC()` which sends no `role` parameter, so
the server treated it as a real agent connection and enabled connection
monitoring. When the helper closed, its monitor asynchronously wrote
`disconnectedAt` to the DB — racing with the real agent's monitor and
transiently marking the agent as disconnected.

The fix uses `ConnectRPCWithRole(ctx, "apptest-manifest")` so the helper
doesn't trigger connection monitoring. The server already has this
role-based distinction for non-agent clients like
`coder-logstream-kube`; the test helper just wasn't using it.

Both issues share this codepath: `setupProxyTest` →
`createWorkspaceWithApps` → the `ConnectRPC` call at `setup.go:518`.
Both test configurations have a non-empty `PrimaryAppHost`, so both
enter the affected block.

This is not masking a product issue — the "disconnected" state was
caused by two competing monitors writing to the same agent DB row, a
scenario that only exists in this test setup. No assertions were
weakened; the proxy still checks real agent connectivity on every
request.

coderd/workspaceapps/apptest/setup.go

@@ -515,7 +515,11 @@ func createWorkspaceWithApps(t *testing.T, client *codersdk.Client, orgID uuid.U
 	primaryAppHost, err := client.AppHost(appHostCtx)
 	require.NoError(t, err)
 	if primaryAppHost.Host != "" {
-		rpcConn, err := agentClient.ConnectRPC(appHostCtx)
+		// Fetch the manifest without marking this short-lived helper
+		// connection as the workspace agent. Closing a monitored RPC
+		// connection races with the real agent startup and can
+		// transiently mark the agent disconnected.
+		rpcConn, err := agentClient.ConnectRPCWithRole(appHostCtx, "apptest-manifest")
 		require.NoError(t, err)
 		aAPI := agentproto.NewDRPCAgentClient(rpcConn)
 		manifest, err := aAPI.GetManifest(appHostCtx, &agentproto.GetManifestRequest{})