refactor(json-rpc-engine): Genericize JsonRpcServer middleware / engine

[rekmarks]

Explanation

As it turns out, the JsonRpcServer needs its middleware / engine to be generic in order to support certain use cases. Without generics, its middleware type needs to be JsonRpcMiddleware<JsonRpcCall>, which prevents constructing engines that only support either requests or notifications.

Enabling this comes at the cost of some type safety, but the server will still handle any errors thrown by its engine due to the latter's lack of support of requests or notifications. The alternative would be to make the server aware of which type of requests it supports at runtime, which seems overkill.

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

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Note

Genericizes JsonRpcServer over middleware/engine types, exports RequestOf/MergedContextOf, updates v2 index exports, adds tests for request/notification-only servers, and clarifies docs with warnings.

• Server (v2/JsonRpcServer)
  • Genericize class and Options over JsonRpcMiddleware; type underlying JsonRpcEngineV2 via RequestOf/MergedContextOf.
  • Accept middleware directly with preserved types; add warnings in JSDoc about request vs. notification handling.
• Types/Exports
  • Export RequestOf and MergedContextOf from v2/JsonRpcEngineV2.
  • Replace wildcard re-export in v2/index.ts with named exports and type re-exports (JsonRpcMiddleware, MiddlewareParams, Next, ResultConstraint).
• Tests
  • Add tests ensuring errors when a server supports only requests or only notifications and receives the wrong kind.
  • Minor wording/import adjustments in JsonRpcServer.test.ts.
• Docs
  • README: refine warning about mixing JsonRpcRequest/JsonRpcNotification middleware; add warning about servers that only accept one kind.
  • Changelog: add PR reference for JsonRpcEngineV2 item.

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