fix(plugins): keep test helpers out of contract barrels (#63311)

Merged via squash.

Prepared head SHA: 769e90c6af
Co-authored-by: altaywtf <9790196+altaywtf@users.noreply.github.com>
Co-authored-by: altaywtf <9790196+altaywtf@users.noreply.github.com>
Reviewed-by: @altaywtf

extensions/slack/src/outbound-payload.test.ts

@@ -1,6 +1,6 @@
 import type { ReplyPayload } from "openclaw/plugin-sdk/reply-runtime";
 import { describe, expect, it } from "vitest";
-import { createSlackOutboundPayloadHarness } from "../contract-api.js";
+import { createSlackOutboundPayloadHarness } from "../test-api.js";
 
 function createHarness(params: {
   payload: ReplyPayload;