[ISSUE #10538] Eliminate SendMessageRequestHeaderV2 intermediate object in client

Client creates SendMessageRequestHeaderV2 per send with 14 field copies. With #10535 FastCodesHeader, SendMessageRequestHeader can encode V2 directly, making the intermediate object unnecessary.

Start by tracing the client send path to SendMessageRequestHeaderV2 and SendMessageRequestHeader, then review the FastCodesHeader change in #10535. Confirm where the 14 field copies occur and how V2 encoding is selected. Done means the intermediate object is no longer created while the client still produces the same V2 request encoding and relevant client tests pass.

Written by the indexing model from the issue text.