refactor: deprecate AIGatewayRoutingEnabled, remove direct chat routing (#26862)

This PR removes the now-dead direct-routing code:

- Deletes the direct routing implementation.
- Collapses the resolvedModelRoute discriminated union into aiGatewayModelRoute.
- Removes the dead providerKeys cascade.
- Deletes the preferredShortTextCandidates quickgen function.
- Simplifies the advisor override error handling.
- Deprecates the AIGatewayRoutingEnabled deployment option. It is now a no-op so as to not break existing deployments on upgrade.

Once direct routing was gone, the AI Gateway became mandatory for chat, which surfaced gaps in how the product behaves with the gateway disabled:

- Exposes ai-gateway-enabled to the frontend via embedded page metadata.
- Disables the chat composer via the existing AgentSetupNotice when the gateway is disabled, for both new and existing chats.
- Fixes nil/typed-nil chatDaemon panics on startup and shutdown when gateway is disabled.
- Fixes chat WebSocket from retrying the still-gated stream endpoint forever when the gateway is disabled.

coderd/coderd_internal_test.go

@@ -8,6 +8,7 @@ import (
 
 	"github.com/go-chi/chi/v5"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestStripSlashesMW(t *testing.T) {
@@ -65,3 +66,15 @@ func TestStripSlashesMW(t *testing.T) {
 		})
 	}
 }
+
+// TestChatDaemonPublishDiffStatusChangeFunc verifies that
+// chatDaemonPublishDiffStatusChangeFunc returns a true nil, not a method
+// value bound to a nil receiver, when chatDaemon is nil. See that function
+// for why the distinction matters. The non-nil path is covered by
+// coderd/exp_chats_test.go.
+func TestChatDaemonPublishDiffStatusChangeFunc(t *testing.T) {
+	t.Parallel()
+
+	fn := chatDaemonPublishDiffStatusChangeFunc(nil)
+	require.Nil(t, fn, "func value must be a true nil, not a bound method on a nil receiver")
+}