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.

cli/testdata/server-config.yaml.golden

@@ -796,9 +796,8 @@ chat:
   # opt-in settings.
   # (default: false, type: bool)
   debugLoggingEnabled: false
-  # Route chat model requests through AI Gateway when both chat routing and AI
-  # Gateway are enabled. Otherwise, chat calls AI providers directly. Pending chats
-  # without API key metadata may need a retry or temporary direct routing.
+  # Deprecated: AI Gateway routing is now the only routing path. Setting this value
+  # has no effect. This option will be removed in a future release.
   # (default: true, type: bool)
   aiGatewayRoutingEnabled: true
 aibridge: