refactor(ai-client): extract shared createLanguageModel helper

De-duplicate the provider switch logic that was identical in chat.ts
and vision.ts into a shared model.ts module.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/ai-client/model.ts

@@ -0,0 +1,23 @@
+import { createAnthropic } from "@ai-sdk/anthropic";
+import { createGoogleGenerativeAI } from "@ai-sdk/google";
+import { createOpenAI } from "@ai-sdk/openai";
+import type { ProviderConfig, ProviderProtocol } from "@infiplot/types";
+import { normalizeBaseUrl } from "./normalizeUrl";
+
+export function resolveProtocol(config: ProviderConfig): ProviderProtocol {
+  return config.provider ?? "openai_compatible";
+}
+
+export function createLanguageModel(config: ProviderConfig, protocol: ProviderProtocol) {
+  const baseURL = normalizeBaseUrl(config.baseUrl, protocol);
+  switch (protocol) {
+    case "anthropic":
+      return createAnthropic({ apiKey: config.apiKey, baseURL })(config.model);
+    case "google":
+      return createGoogleGenerativeAI({ apiKey: config.apiKey, baseURL })(config.model);
+    case "openai_compatible":
+    case "openai":
+    default:
+      return createOpenAI({ apiKey: config.apiKey, baseURL }).chat(config.model);
+  }
+}