test(react-core): move threadId-propagation test out of the hooks dir

The previous regression (CopilotKit#5041, shared root cause with CopilotKit#4739) slipped through
because the original coverage (use-agent-thread-isolation.test.tsx) lived
next to the per-thread-cloning feature and was deleted alongside it when
cloning was reverted. The invariant outlived the feature but the tests didn't.

Relocate to packages/react-core/src/__tests__/ and rename as a contract test
so future implementation swaps (cloning, effect, prop drilling, context) keep
it in scope. Tightened the header docstring to spell out the invariant and the
reason for the placement.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: BenTaylorDev

…tests__/use-agent-threadid-sync.test.tsx …_/threadid-propagation.contract.test.tsxpackages/react-core/src/v2/hooks/__tests__/use-agent-threadid-sync.test.tsx renamed to packages/react-core/src/__tests__/threadid-propagation.contract.test.tsx packages/react-core/src/v2/hooks/__tests__/use-agent-threadid-sync.test.tsx renamed to packages/react-core/src/__tests__/threadid-propagation.contract.test.tsx

@@ -2,29 +2,38 @@ import React from "react";
 import { render, act } from "@testing-library/react";
 import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
 import type { AbstractAgent } from "@ag-ui/client";
-import { useCopilotKit } from "../../context";
-import { useAgent } from "../use-agent";
-import { CopilotChatConfigurationProvider } from "../../providers/CopilotChatConfigurationProvider";
+import { useCopilotKit } from "../v2/context";
+import { useAgent } from "../v2/hooks/use-agent";
+import { CopilotChatConfigurationProvider } from "../v2/providers/CopilotChatConfigurationProvider";
 import { CopilotKitCoreRuntimeConnectionStatus } from "@copilotkit/core";
 
-vi.mock("../../context", () => ({
+vi.mock("../v2/context", () => ({
   useCopilotKit: vi.fn(),
 }));
 
 const mockUseCopilotKit = useCopilotKit as ReturnType<typeof vi.fn>;
 
 /**
- * Regression coverage for issue #5041 (and the same root cause behind #4739):
- * <CopilotKit threadId={x}> flows into CopilotChatConfigurationProvider, but
- * the underlying ProxiedCopilotRuntimeAgent was never having `agent.threadId`
- * set from that value. The AbstractAgent constructor auto-mints a UUID, so
- * outbound /agent/run, /agent/connect, /agent/stop requests carried a UUID
- * that diverged from what app code read via useThreads/useCopilotChatConfig.
+ * Contract-level regression coverage for the threadId-propagation invariant.
  *
- * The fix wires useAgent to sync `agent.threadId` from the chat configuration
- * when `hasExplicitThreadId` is true. This file pins that behavior so the
- * propagation can't silently regress again (as it did when per-thread cloning
- * was removed in May 2026).
+ * Lives at the package root (src/__tests__) rather than next to any one
+ * implementation site on purpose: the previous regression (issue #5041, root
+ * cause shared with #4739) slipped through because the original coverage
+ * (use-agent-thread-isolation.test.tsx, 333 lines) lived next to the
+ * per-thread-cloning feature and was deleted alongside it in May 2026 when
+ * cloning was reverted. The threadId-propagation invariant survived the
+ * feature change but its tests didn't.
+ *
+ * The invariant under test: a caller-supplied threadId (via <CopilotKit>,
+ * <ThreadsProvider>, or <CopilotChatConfigurationProvider>) MUST end up on
+ * the underlying agent's `threadId` field, because ProxiedCopilotRuntimeAgent
+ * uses that field to address /agent/run, /agent/connect, /agent/stop. Without
+ * it the agent ships its own auto-minted UUID and the backend sees a different
+ * thread than the app code reads via useThreads/useCopilotChatConfiguration.
+ *
+ * This file should outlive any specific implementation strategy (cloning,
+ * effect-based sync, prop drilling, context bridge). If you change the
+ * mechanism, keep this contract.
  */
 describe("useAgent → agent.threadId sync from chat configuration", () => {
   let mockCopilotkit: {