docs: surface Kijiji AND-matching behavior in tool, API, and MCP responses

Kijiji zero-result queries (e.g. 'macbook air m1 apple silicon') are
confusing because the failure mode is non-obvious. Surface the root
cause everywhere the caller can see it:
- MCP tool description warns about AND-matching and gives a concrete
  before/after example
- API 404 body includes the actionable hint via emptySearchResponse(hint)
- Core scraper logs the built URL and tip on page-1 zero results
- MCP handler unwraps the API message field so the hint reaches the LLM

packages/api-server/test/routes.test.ts

@@ -394,7 +394,8 @@ describe("API routes", () => {
 
     expect(response.status).toBe(404);
     const body = await response.json();
-    expect(body.message).toBe("Search didn't return any results!");
+    expect(body.message).toStartWith("Search didn't return any results!");
+    expect(body.message).toContain("Kijiji matches ALL words");
   });
 
   test("ebayRoute forwards maxItems to core in default mode", async () => {