Lift app-server JSON-RPC error handling to request boundary

[pakrym-oai]

Why

App-server request handling had a lot of repeated JSON-RPC error construction and one-off send_error/return branches. This made small handlers noisy and pushed error response details into leaf code that otherwise only needed to validate input or call the underlying API.

What Changed

• Added shared JSON-RPC error constructors in codex-rs/app-server/src/error_code.rs.
• Lifted straightforward request result emission into codex-rs/app-server/src/message_processor.rs so response/error dispatch happens at the request boundary.
• Reused the result helpers across command exec, config, filesystem, device-key, external-agent config, fs-watch, and outgoing-message paths.
• Removed leaf wrapper handlers where the method body was only forwarding to a response helper.
• Returned request validation errors upward in the simple cases instead of sending an error locally and immediately returning.

Verification

• cargo test -p codex-app-server --lib command_exec::tests
• cargo test -p codex-app-server --lib outgoing_message::tests
• cargo test -p codex-app-server --lib in_process::tests
• cargo test -p codex-app-server --test all v2::fs
• cargo test -p codex-app-server --test all v2::config_rpc
• cargo test -p codex-app-server --test all v2::external_agent_config
• cargo test -p codex-app-server --test all v2::initialize
• just fix -p codex-app-server
• git diff --check

Note: full cargo test -p codex-app-server was attempted and stopped in message_processor::tracing_tests::turn_start_jsonrpc_span_parents_core_turn_spans with a stack overflow after unrelated tests had already passed.