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/workspaceagentsrpc.go

@@ -143,6 +143,17 @@ func (api *API) workspaceAgentRPC(rw http.ResponseWriter, r *http.Request) {
 			slog.F("role", role))
 	}
 
+	// api.chatDaemon is a *chatd.Server that stays nil when AI Gateway is
+	// disabled. Assigning a nil *chatd.Server directly to the
+	// interface-typed ContextDirtyMarker field below would produce a
+	// non-nil interface value (a typed nil), defeating agentapi's own
+	// "if DirtyMarker != nil" check and panicking on first use. Only set
+	// the field when there's a real chat daemon to call into.
+	var contextDirtyMarker agentapi.ContextDirtyMarker
+	if api.chatDaemon != nil {
+		contextDirtyMarker = api.chatDaemon
+	}
+
 	agentAPI := agentapi.New(agentapi.Options{
 		AgentID:           workspaceAgent.ID,
 		OwnerID:           workspace.OwnerID,
@@ -180,9 +191,7 @@ func (api *API) workspaceAgentRPC(rw http.ResponseWriter, r *http.Request) {
 
 		// Optional:
 		UpdateAgentMetricsFn: api.UpdateAgentMetrics,
-		// chatDaemon is always constructed (only its worker is gated), so
-		// this is non-nil; agentapi treats a nil marker as "chatd absent".
-		ContextDirtyMarker: api.chatDaemon,
+		ContextDirtyMarker:   contextDirtyMarker,
 	}, workspace, workspaceAgent)
 
 	streamID := tailnet.StreamID{