[Feature] Add jsonrpc-test-utils module with reusable JsonRpcAssertions

[limehee]

Problem statement

JSON-RPC tests repeatedly assert the same protocol patterns (success response shape, error code expectations, notification no-response behavior). This leads to duplicated assertions and lower readability across modules and downstream adopters.

Proposed solution

Add a dedicated jsonrpc-test-utils module with reusable assertion helpers, for example:

• assertSuccess(response)
• assertErrorCode(response, expectedCode)
• assertNotificationNoResponse(dispatchResult)
• optional batch helpers for count and per-id checks

Design goals:

• concise and intention-revealing test code
• reusable across jsonrpc-core, spring-webmvc, samples, and external projects
• no runtime dependency impact for production artifacts

Alternatives considered

• Continue using local ad-hoc assertion methods in each test class
• Publish only copy-paste snippets in docs

These options keep duplication and inconsistency.

JSON-RPC behavior impact

No protocol behavior change. This is test support only.

Additional context

• Keep the module lightweight and JUnit 5 friendly.
• Provide examples in sample tests.
• Ensure assertions cover both single and batch response scenarios.