## 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.
### Summary
Add the v2 app-server filesystem watch RPCs and notifications, wire them
through the message processor, and implement connection-scoped watches
with notify-backed change delivery. This also updates the schema
fixtures, app-server documentation, and the v2 integration coverage for
watch and unwatch behavior.
This allows clients to efficiently watch for filesystem updates, e.g. to
react on branch changes.
### Testing
- exercise watch lifecycles for directory changes, atomic file
replacement, missing-file targets, and unwatch cleanup
