Improve chat navigation, model selection, and streaming

lib/chat/messages.ts

@@ -0,0 +1,15 @@
+import type { UIMessage } from "ai";
+
+type MessagePart = UIMessage["parts"][number];
+type ReasoningPart = Extract<MessagePart, { type: "reasoning" }>;
+
+export function getTextContent(parts: MessagePart[]): string {
+  return parts
+    .filter((p) => p.type === "text")
+    .map((p) => p.text || "")
+    .join("");
+}
+
+export function getReasoningPart(parts: MessagePart[]): ReasoningPart | undefined {
+  return parts.find((p): p is ReasoningPart => p.type === "reasoning");
+}