Fix JSON-RPC notification responses

[GhostStar]

Summary

• suppress responses for JSON-RPC requests whose id member is absent
• keep explicit id: null distinct and return a valid response for it
• keep executing notifications while returning no WebSocket frame or HTTP 204
• filter notifications from mixed batches and preserve response ordering
• allow the HTTP writer to emit an empty 204 response safely

Root cause

viem sends this notification every 30 seconds as its WebSocket keepalive:

{"jsonrpc":"2.0","method":"net_version","params":[]}

NodeCore mapped the missing client ID to an empty byte slice and serialized a response as:

{"id":,"jsonrpc":"2.0","result":"4663"}

That invalid frame makes viem fail JSON parsing and close the shared subscription socket. JSON-RPC 2.0 notifications must not receive a response.

Tests

• handler tests for missing IDs, explicit null IDs, and mixed batch ordering
• HTTP integration test verifies 204 with an empty body
• WebSocket integration test verifies silence followed by a successful regular response on the same connection
• go test ./internal/server/http_server -count=1
• make build

The full suite passes except for the pre-existing timing-sensitive TestEngineSlowConsumerDoesNotAffectFastPeer when run under full-suite load; it passes independently for 3 consecutive runs.