fix: Clone JsonRpcEngineV2 results to prevent returning frozen objects

[FrederikBolding]

Explanation

JsonRpcEngineV2 internally freezes request and result to prevent middlewares from modifying the objects. This implementation detail has leaked into the result returned from handle which does not seem expected and is causing problems downstream. To work around this issue, this PR adds logic to clone the result before returning it, undoing any frozen objects.

References

https://consensyssoftware.atlassian.net/browse/WPC-510

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Note

Medium Risk
Changes the runtime behavior of JsonRpcEngineV2.handle() by returning deep-cloned results (new object identities) instead of the internally frozen objects, which may affect consumers relying on referential equality or immutability and adds cloning overhead.

Overview
JsonRpcEngineV2.handle() now deep-clones the final middleware result before returning it, so callers no longer receive the engine’s internally frozen objects (and the return type is no longer Readonly).

Adds coverage to ensure returned objects (including nested objects) are not frozen, updates downstream cache middleware tests to use toStrictEqual instead of reference equality, and records the fix in the json-rpc-engine changelog.

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[mikesposito]

LGTM!