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-handler.process.test.ts

@@ -59,6 +59,7 @@ async function createBaseContext(overrides: Record<string, unknown> = {}) {
       timestamp: new Date().toISOString(),
       attachments: [],
     },
+    messageChannelId: "c1",
     author: {
       id: "U1",
       username: "alice",
@@ -130,4 +131,25 @@ describe("processDiscordMessage ack reactions", () => {
 
     expect(reactMessageDiscord).toHaveBeenCalledWith("c1", "m1", "👀", { rest: {} });
   });
+
+  it("uses preflight-resolved messageChannelId when message.channelId is missing", async () => {
+    const ctx = await createBaseContext({
+      message: {
+        id: "m1",
+        timestamp: new Date().toISOString(),
+        attachments: [],
+      },
+      messageChannelId: "fallback-channel",
+      shouldRequireMention: true,
+      effectiveWasMentioned: true,
+      sender: { label: "user" },
+    });
+
+    // oxlint-disable-next-line typescript/no-explicit-any
+    await processDiscordMessage(ctx as any);
+
+    expect(reactMessageDiscord).toHaveBeenCalledWith("fallback-channel", "m1", "👀", {
+      rest: {},
+    });
+  });
 });