Fix malformed JSON-RPC envelope handling

[rayzhudev]

Summary

• reject unknown fields on JSON-RPC lite envelope structs so hybrid request/response payloads are not silently accepted
• add JSON-RPC lite serialization/deserialization tests, including malformed envelope regressions
• update exported JSON schema fixtures for the stricter envelope objects

Tests

• cargo test -p codex-app-server-protocol jsonrpc_lite
• just fmt
• just fix -p codex-app-server-protocol
• jq empty codex-rs/app-server-protocol/schema/json/JSONRPCError.json codex-rs/app-server-protocol/schema/json/JSONRPCErrorError.json codex-rs/app-server-protocol/schema/json/JSONRPCMessage.json codex-rs/app-server-protocol/schema/json/JSONRPCNotification.json codex-rs/app-server-protocol/schema/json/JSONRPCRequest.json codex-rs/app-server-protocol/schema/json/JSONRPCResponse.json codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

Notes

• just write-app-server-schema was attempted but failed because the machine had only ~116 MiB free and rustc reported No space left on device; the deterministic JSON schema changes were applied manually and validated with jq.