fix: guard against JsonNull when parsing response error field

[RainYuY]

Description

Root Cause

The serializer and deserializer are inconsistent:

• A2AErrorTypeAdapter.write() emits "error": null for null errors
• parseResponseBody() assumes the key's presence implies a non-null object

The correct long-term fix is to also stop emitting "error": null in
A2AErrorTypeAdapter.write(), but this client-side guard is sufficient to
prevent the crash.

Impact

Without this fix, every non-streaming A2A call fails with ClassCastException
regardless of whether the response is actually an error.

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request updates the JSON-RPC response parsing in JSONRPCUtils.java to handle cases where the 'error' field is present but null. The review feedback suggests optimizing this logic by retrieving the 'error' element once to avoid multiple lookups and recommends applying this pattern consistently to other similar methods like parseResponseEvent.

[gemini-code-assist]

The current implementation performs multiple lookups for the 'error' field. A more efficient approach is to retrieve the element once. Since the JSON-RPC specification defines the error field as an object, we can rely on this constraint and avoid additional defensive type checks for primitives. This same pattern should be applied to parseResponseEvent at line 293.

        JsonElement errorNode = jsonRpc.get("error");
        if (errorNode != null && !errorNode.isJsonNull()) {
            return parseError(errorNode.getAsJsonObject(), id, method);
        }

References

1. When handling return types constrained by a formal specification, it is acceptable to rely on the specification's constraints rather than adding defensive checks for types that the specification explicitly excludes.