fix(json-rpc-engine): Export MiddlewareContext class (#6971)

## Explanation

Export the `MiddlewareContext` class in addition to its type.

## References

N/A

## Checklist

- [x] I've updated the test suite for new or updated code as appropriate
- [x] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [x] I've communicated my changes to consumers by [updating changelogs
for packages I've
changed](https://github.com/MetaMask/core/tree/main/docs/contributing.md#updating-changelogs),
highlighting breaking changes as necessary
- [x] I've prepared draft pull requests for clients and consumer
packages to resolve any breaking changes

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Exports the MiddlewareContext class from json-rpc-engine/v2 and
updates tests and changelog references.
> 
> - **Exports (`packages/json-rpc-engine/src/v2/index.ts`)**:
> - Add `MiddlewareContext` class to v2 public exports; keep
`EmptyContext` as type-only export.
> - **Tests (`packages/json-rpc-engine/src/v2/index.test.ts`)**:
>   - Update export snapshot (sorted) to include `MiddlewareContext`.
> - **Changelog (`packages/json-rpc-engine/CHANGELOG.md`)**:
>   - Add PR reference `#6971` to `JsonRpcEngineV2` entry.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
e8f2750. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: rekmarks

packages/json-rpc-engine/CHANGELOG.md

@@ -9,7 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
-- `JsonRpcEngineV2` ([#6176](https://github.com/MetaMask/core/pull/6176))
+- `JsonRpcEngineV2` ([#6176](https://github.com/MetaMask/core/pull/6176), [#6971](https://github.com/MetaMask/core/pull/6971))
   - This is a complete rewrite of `JsonRpcEngine`, intended to replace the original implementation.
     See the readme for details.
 

packages/json-rpc-engine/src/v2/index.test.ts

@@ -2,15 +2,16 @@ import * as allExports from '.';
 
 describe('@metamask/json-rpc-engine/v2', () => {
   it('has expected JavaScript exports', () => {
-    expect(Object.keys(allExports)).toMatchInlineSnapshot(`
+    expect(Object.keys(allExports).sort()).toMatchInlineSnapshot(`
       Array [
+        "JsonRpcEngineError",
+        "JsonRpcEngineV2",
+        "JsonRpcServer",
+        "MiddlewareContext",
         "asLegacyMiddleware",
         "getUniqueId",
-        "JsonRpcServer",
         "isNotification",
         "isRequest",
-        "JsonRpcEngineError",
-        "JsonRpcEngineV2",
       ]
     `);
   });

packages/json-rpc-engine/src/v2/index.ts

@@ -2,7 +2,8 @@ export { asLegacyMiddleware } from './asLegacyMiddleware';
 export { getUniqueId } from '../getUniqueId';
 export * from './JsonRpcEngineV2';
 export { JsonRpcServer } from './JsonRpcServer';
-export type { MiddlewareContext, EmptyContext } from './MiddlewareContext';
+export { MiddlewareContext } from './MiddlewareContext';
+export type { EmptyContext } from './MiddlewareContext';
 export { isNotification, isRequest, JsonRpcEngineError } from './utils';
 export type {
   Json,