Lift app-server JSON-RPC error handling to request boundary (#19484)

## 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.

codex-rs/app-server/src/fs_watch.rs

@@ -1,4 +1,4 @@
-use crate::fs_api::invalid_request;
+use crate::error_code::invalid_request;
 use crate::outgoing_message::ConnectionId;
 use crate::outgoing_message::OutgoingMessageSender;
 use codex_app_server_protocol::FsChangedNotification;