fix(slack): map legacy streaming=false to off (openclaw#26020) thanks @chilu18

Verified:
- pnpm build
- pnpm check
- pnpm test:macmini

Co-authored-by: chilu18 <7957943+chilu18@users.noreply.github.com>
Co-authored-by: Tak Hoffman <781889+Takhoffman@users.noreply.github.com>

src/slack/stream-mode.test.ts

@@ -40,12 +40,17 @@ describe("resolveSlackStreamingConfig", () => {
     });
   });
 
-  it("moves legacy streaming boolean to native streaming toggle", () => {
+  it("maps legacy streaming booleans to unified mode and native streaming toggle", () => {
     expect(resolveSlackStreamingConfig({ streaming: false })).toEqual({
-      mode: "partial",
+      mode: "off",
       nativeStreaming: false,
       draftMode: "replace",
     });
+    expect(resolveSlackStreamingConfig({ streaming: true })).toEqual({
+      mode: "partial",
+      nativeStreaming: true,
+      draftMode: "replace",
+    });
   });
 
   it("accepts unified enum values directly", () => {