Slack: add media block fallback text handling

src/slack/blocks-fallback.test.ts

@@ -0,0 +1,31 @@
+import { describe, expect, it } from "vitest";
+import { buildSlackBlocksFallbackText } from "./blocks-fallback.js";
+
+describe("buildSlackBlocksFallbackText", () => {
+  it("prefers header text", () => {
+    expect(
+      buildSlackBlocksFallbackText([
+        { type: "header", text: { type: "plain_text", text: "Deploy status" } },
+      ] as never),
+    ).toBe("Deploy status");
+  });
+
+  it("uses image alt text", () => {
+    expect(
+      buildSlackBlocksFallbackText([
+        { type: "image", image_url: "https://example.com/image.png", alt_text: "Latency chart" },
+      ] as never),
+    ).toBe("Latency chart");
+  });
+
+  it("uses generic defaults for file and unknown blocks", () => {
+    expect(
+      buildSlackBlocksFallbackText([
+        { type: "file", source: "remote", external_id: "F123" },
+      ] as never),
+    ).toBe("Shared a file");
+    expect(buildSlackBlocksFallbackText([{ type: "divider" }] as never)).toBe(
+      "Shared a Block Kit message",
+    );
+  });
+});