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

[rekmarks]

Explanation

Ensures that the MiddlewareContext class in addition to type is exported from json-rpc-engine/v2.

References

N/A

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed, highlighting breaking changes as necessary
• I've prepared draft pull requests for clients and consumer packages to resolve any breaking changes

---

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.

^{Written by Cursor Bugbot for commit e8f2750. This will update automatically on new commits. Configure here.}