chore(app-server-protocol): split v2 API definitions into modules (#21251)

## Why

`codex-rs/app-server-protocol/src/protocol/v2.rs` had grown into a
single ~12k-line definition file for the entire app-server v2 API.

This is purely a mechanical refactor to break up the monolithic `v2.rs`
file that contains all app-server API v2 types into more modular files,
grouped by resource (e.g. account, thread, turn, etc.).

`just write-app-server-schema` shows no real changes, so we can be sure
that this is purely an internal organizational change.

## What changed

- Replaced the monolithic `protocol/v2.rs` with a `protocol/v2/` module
tree and a small `mod.rs` that only declares and reexports modules.
- Grouped v2 API definitions by conceptual owner, including `account`,
`apps`, `collaboration_mode`, `command_exec`, `config`, `device_key`,
`experimental_feature`, `feedback`, `fs`, `hook`, `item`, `mcp`,
`model`, `notification`, `permissions`, `plugin`, `process`, `realtime`,
`review`, `thread`, `thread_data`, `turn`, and `windows_sandbox`.
- Moved v2 tests into `protocol/v2/tests.rs` so `mod.rs` stays small.
- Kept shared protocol helpers in `protocol/v2/shared.rs`, including the
enum mirroring macro and common cross-resource types.
- Co-located resource-specific notifications and server-request payloads
with the modules that own those resources.
- Regenerated app-server protocol schema fixtures. The schema diffs are
non-semantic newline-only changes after the refactor.

## Verification

- `cargo check -p codex-app-server-protocol`
- `cargo test -p codex-app-server-protocol`
- `just write-app-server-schema`

codex-rs/app-server-protocol/src/protocol/v2/mod.rs

@@ -0,0 +1,53 @@
+mod shared;
+
+mod account;
+mod apps;
+mod collaboration_mode;
+mod command_exec;
+mod config;
+mod device_key;
+mod experimental_feature;
+mod feedback;
+mod fs;
+mod hook;
+mod item;
+mod mcp;
+mod model;
+mod notification;
+mod permissions;
+mod plugin;
+mod process;
+mod realtime;
+mod review;
+mod thread;
+mod thread_data;
+mod turn;
+mod windows_sandbox;
+
+pub use account::*;
+pub use apps::*;
+pub use collaboration_mode::*;
+pub use command_exec::*;
+pub use config::*;
+pub use device_key::*;
+pub use experimental_feature::*;
+pub use feedback::*;
+pub use fs::*;
+pub use hook::*;
+pub use item::*;
+pub use mcp::*;
+pub use model::*;
+pub use notification::*;
+pub use permissions::*;
+pub use plugin::*;
+pub use process::*;
+pub use realtime::*;
+pub use review::*;
+pub use shared::*;
+pub use thread::*;
+pub use thread_data::*;
+pub use turn::*;
+pub use windows_sandbox::*;
+
+#[cfg(test)]
+mod tests;