[Task]: Update JSON-RPC error format according to the latest spec changes

[ishymko]

There was a hot-fix in the 1.0 A2A spec done in a2aproject/A2A#1627, there is a statement (a2aproject/A2A#1627 (comment)) that this change is effective immediately for 1.0 spec, so should be applied here.

See a2aproject/a2a-go#327 for reference.