fix(discord): preserve channel session keys via channel_id fallbacks (#17622)

* fix(discord): preserve channel session keys via channel_id fallbacks

* docs(changelog): add discord session continuity note

* Tests: cover discord channel_id fallback

---------

Co-authored-by: Shadow <hi@shadowing.dev>

src/discord/monitor/message-utils.test.ts

@@ -0,0 +1,38 @@
+import type { Message } from "@buape/carbon";
+import { describe, expect, it } from "vitest";
+import { resolveDiscordMessageChannelId } from "./message-utils.js";
+
+function asMessage(payload: Record<string, unknown>): Message {
+  return payload as unknown as Message;
+}
+
+describe("resolveDiscordMessageChannelId", () => {
+  it("uses message.channelId when present", () => {
+    const channelId = resolveDiscordMessageChannelId({
+      message: asMessage({ channelId: " 123 " }),
+    });
+    expect(channelId).toBe("123");
+  });
+
+  it("falls back to message.channel_id", () => {
+    const channelId = resolveDiscordMessageChannelId({
+      message: asMessage({ channel_id: " 234 " }),
+    });
+    expect(channelId).toBe("234");
+  });
+
+  it("falls back to message.rawData.channel_id", () => {
+    const channelId = resolveDiscordMessageChannelId({
+      message: asMessage({ rawData: { channel_id: "456" } }),
+    });
+    expect(channelId).toBe("456");
+  });
+
+  it("falls back to eventChannelId and coerces numeric values", () => {
+    const channelId = resolveDiscordMessageChannelId({
+      message: asMessage({}),
+      eventChannelId: 789,
+    });
+    expect(channelId).toBe("789");
+  });
+});