[WIP] Add server id field to RPC message logger

[Copilot]

Plan: Change server_id to server in RPC message logger

• Update JSONLRPCMessage struct to use "server" JSON tag instead of "server_id"
• Update test to verify "server" field instead of "server_id"
• Run unit tests to validate changes
• Run full test suite with make agent-finished

Original prompt

The RPC message logger should include the server id in the JSON payload as a "server" field

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