feat(ai): add shared contentText utility (#6840)

Co-authored-by: Armin Ronacher <armin.ronacher@active-4.com>

packages/ai/test/text.test.ts

@@ -0,0 +1,33 @@
+import { describe, expect, it } from "vitest";
+import { type AssistantMessage, contentText, type ToolResultMessage } from "../src/index.ts";
+
+const content: AssistantMessage["content"] = [
+	{ type: "thinking", thinking: "reasoning" },
+	{ type: "text", text: "first" },
+	{ type: "toolCall", id: "1", name: "read", arguments: {} },
+	{ type: "text", text: "second" },
+];
+
+describe("contentText", () => {
+	it("extracts assistant text blocks", () => {
+		expect(contentText(content)).toBe("first\nsecond");
+	});
+
+	it("supports custom separators", () => {
+		expect(contentText(content, "")).toBe("firstsecond");
+	});
+
+	it("passes string content through", () => {
+		expect(contentText("hello")).toBe("hello");
+	});
+
+	it("extracts text from tool-result content", () => {
+		const toolResultContent: ToolResultMessage["content"] = [
+			{ type: "text", text: "first" },
+			{ type: "image", data: "...", mimeType: "image/png" },
+			{ type: "text", text: "second" },
+		];
+
+		expect(contentText(toolResultContent, "")).toBe("firstsecond");
+	});
+});