Fix RPC writeJSON superfluous WriteHeader on encode failure

[swetjen]

Summary

• fix rpc.writeJSON to avoid calling http.Error after headers are already written
• keep response write path single-header even when JSON encode/write fails (e.g. broken pipe / client disconnect)
• add regression test asserting only one WriteHeader call occurs on encode failure
• update changelog

Why

writeJSON previously wrote status first, then on encode failure attempted http.Error, which triggers Go's superfluous response.WriteHeader warning.

Testing

• go test ./rpc -run 'TestWriteJSONEncodeFailureDoesNotWriteHeaderTwice|TestRPCHandleOK|TestRPCHandleInvalid'
• go test ./httpapi

Note: full go test ./rpc currently includes an unrelated existing failure in TestRPCPgtypeOpenAPIAndClients in this environment.