[Bug]: inconsistent snake_case usage for REST

[ishymko]

What happened?

Tests for REST transport use snake_case for fields by converting request payloads via convert_a2a_message_to_protobuf_json. Response conversion uses convert_protobuf_response_to_a2a_json as all the validations operate on camelCase. However this isn't done in send_streaming_message while assertions still use camelCase (i.e. here).

The proposal is to use camelCase due to:

1. v0.3.0 doesn't mention casing explicitly for REST, however in this example response payload in the REST tab is shown in camelCase.

2. v1.0 draft explicitly specifies using camelCase everywhere: 5.5. JSON Field Naming Convention.

3. SDKs already use camelCase (at lest it's the case for Java, Python and Go).

Relevant log output

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