chore (rsc): move HANGING_STREAM_WARNING_TIME constant into @ai-sdk/rsc package (#6195)

## Background

The constant `HANGING_STREAM_WARNING_TIME` is only moved `@ai-sdk/rsc`
after the removal of `StreamData`.

## Summary

Move the `HANGING_STREAM_WARNING_TIME` constant into `@ai-sdk/rsc`

Author: lgrammel

.changeset/chatty-ladybugs-nail.md

@@ -0,0 +1,6 @@
+---
+'@ai-sdk/rsc': patch
+'ai': patch
+---
+
+chore (rsc): move HANGING_STREAM_WARNING_TIME constant into @ai-sdk/rsc package

packages/ai/internal/index.ts

@@ -8,5 +8,3 @@ export {
   createCallbacksTransformer,
   type StreamCallbacks,
 } from '../streams/stream-callbacks';
-
-export * from '../util/constants';

packages/rsc/src/streamable-ui/create-streamable-ui.tsx

@@ -1,4 +1,4 @@
-import { HANGING_STREAM_WARNING_TIME_MS } from 'ai/internal';
+import { HANGING_STREAM_WARNING_TIME_MS } from '../util/constants';
 import { createResolvablePromise } from '../util/create-resolvable-promise';
 import { createSuspendedChunk } from './create-suspended-chunk';
 

packages/rsc/src/streamable-value/create-streamable-value.ts

@@ -1,4 +1,4 @@
-import { HANGING_STREAM_WARNING_TIME_MS } from 'ai/internal';
+import { HANGING_STREAM_WARNING_TIME_MS } from '../util/constants';
 import { createResolvablePromise } from '../util/create-resolvable-promise';
 import {
   STREAMABLE_VALUE_TYPE,