refactor: simpler streaming preview and small AI SDK cleanups

- useChat throttles streamed message updates (experimental_throttle,
  150 ms), replacing the two hand-written 150 ms timers of the
  display_diagram and edit_diagram previews (94 lines less). The preview
  now only runs while the input streams; once it is complete the tool
  handler takes over, so a queued preview can no longer redraw an edit
  the handler rejected and rolled back. Measured on a streamed 60-cell
  diagram: 41 redraws at least 97 ms apart, before 37 with gaps down to
  48 ms
- The diagram check endpoint uses streamText with Output.object instead
  of the deprecated streamObject, and returns its fixed result as a plain
  text response; new route test
- Import createGateway/gateway from ai and drop the direct
  @ai-sdk/gateway dependency
- The per-request message structure logs only print with
  DEBUG_LLM_PAYLOAD=true
- Remove an empty onFinish callback

package.json

@@ -35,7 +35,6 @@
         "@ai-sdk/anthropic": "^3.0.127",
         "@ai-sdk/azure": "^3.0.133",
         "@ai-sdk/deepseek": "^2.0.71",
-        "@ai-sdk/gateway": "^3.0.209",
         "@ai-sdk/google": "^3.0.130",
         "@ai-sdk/google-vertex": "^4.0.210",
         "@ai-sdk/openai": "^3.0.124",