test(media-dedup): add missing coverage for Discord media dedup wiring

Cover three integration points where media dedup could silently regress:
- trimMessagingToolSent FIFO cap at 200 entries
- buildReplyPayloads media filter wiring (new test file)
- followup-runner messagingToolSentMediaUrls filtering

src/auto-reply/reply/agent-runner-payloads.test.ts

@@ -0,0 +1,46 @@
+import { describe, expect, it } from "vitest";
+import { buildReplyPayloads } from "./agent-runner-payloads.js";
+
+const baseParams = {
+  isHeartbeat: false,
+  didLogHeartbeatStrip: false,
+  blockStreamingEnabled: false,
+  blockReplyPipeline: null,
+  replyToMode: "off" as const,
+};
+
+describe("buildReplyPayloads media filter integration", () => {
+  it("strips media URL from payload when in messagingToolSentMediaUrls", () => {
+    const { replyPayloads } = buildReplyPayloads({
+      ...baseParams,
+      payloads: [{ text: "hello", mediaUrl: "file:///tmp/photo.jpg" }],
+      messagingToolSentMediaUrls: ["file:///tmp/photo.jpg"],
+    });
+
+    expect(replyPayloads).toHaveLength(1);
+    expect(replyPayloads[0].mediaUrl).toBeUndefined();
+  });
+
+  it("preserves media URL when not in messagingToolSentMediaUrls", () => {
+    const { replyPayloads } = buildReplyPayloads({
+      ...baseParams,
+      payloads: [{ text: "hello", mediaUrl: "file:///tmp/photo.jpg" }],
+      messagingToolSentMediaUrls: ["file:///tmp/other.jpg"],
+    });
+
+    expect(replyPayloads).toHaveLength(1);
+    expect(replyPayloads[0].mediaUrl).toBe("file:///tmp/photo.jpg");
+  });
+
+  it("applies media filter after text filter", () => {
+    const { replyPayloads } = buildReplyPayloads({
+      ...baseParams,
+      payloads: [{ text: "hello world!", mediaUrl: "file:///tmp/photo.jpg" }],
+      messagingToolSentTexts: ["hello world!"],
+      messagingToolSentMediaUrls: ["file:///tmp/photo.jpg"],
+    });
+
+    // Text filter removes the payload entirely (text matched), so nothing remains.
+    expect(replyPayloads).toHaveLength(0);
+  });
+});