Gateway/Dashboard: surface config validation issues (#42664)

Merged via squash.

Prepared head SHA: 43f66cdcf0
Co-authored-by: huntharo <5617868+huntharo@users.noreply.github.com>
Co-authored-by: huntharo <5617868+huntharo@users.noreply.github.com>
Reviewed-by: @huntharo

src/gateway/server.config-patch.test.ts

@@ -72,6 +72,38 @@ describe("gateway config methods", () => {
     expect(res.payload?.config).toBeTruthy();
   });
 
+  it("returns config.set validation details in the top-level error message", async () => {
+    const current = await rpcReq<{
+      hash?: string;
+    }>(requireWs(), "config.get", {});
+    expect(current.ok).toBe(true);
+    expect(typeof current.payload?.hash).toBe("string");
+
+    const res = await rpcReq<{
+      ok?: boolean;
+      error?: {
+        message?: string;
+      };
+    }>(requireWs(), "config.set", {
+      raw: JSON.stringify({ gateway: { bind: 123 } }),
+      baseHash: current.payload?.hash,
+    });
+    const error = res.error as
+      | {
+          message?: string;
+          details?: {
+            issues?: Array<{ path?: string; message?: string }>;
+          };
+        }
+      | undefined;
+
+    expect(res.ok).toBe(false);
+    expect(error?.message ?? "").toContain("invalid config:");
+    expect(error?.message ?? "").toContain("gateway.bind");
+    expect(error?.message ?? "").toContain("allowed:");
+    expect(error?.details?.issues?.[0]?.path).toBe("gateway.bind");
+  });
+
   it("returns a path-scoped config schema lookup", async () => {
     const res = await rpcReq<{
       path: string;