[codex] Split app-server request processors (#20940)

## Why

The app-server request path had grown around a large
`CodexMessageProcessor` plus separate API wrapper/helper modules. That
made the dependency graph hard to see and forced unrelated request
families to share broad processor state.

This PR makes the split mechanical and command-prefix oriented so
request families own only the dependencies they use.

## What changed

- Replaced `CodexMessageProcessor` with command-prefix request
processors under `app-server/src/request_processors/`.
- Removed the old config, device-key, external-agent-config, and fs API
wrapper files by moving their API handling into processors.
- Split apps, plugins, marketplace, catalog, account, MCP, command exec,
fs, git, feedback, thread, turn, thread goals, and Windows sandbox
handling into dedicated processors.
- Kept shared lifecycle, summary conversion, token usage replay, and
shared error mapping only where multiple processors use them; single-use
helpers were inlined into their owning processor.
- Removed the fallback processor path and moved processor tests to
`_tests` files.

## Validation

- `cargo test -p codex-app-server`
- `cargo check -p codex-app-server`
- `just fix -p codex-app-server`

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

@@ -804,7 +804,7 @@ mod tests {
     }
 
     #[tokio::test]
-    async fn in_process_allows_device_key_requests_to_reach_device_key_api() {
+    async fn in_process_allows_device_key_requests_to_reach_device_key_processor() {
         let client = start_test_client(SessionSource::Cli).await;
         const MALFORMED_KEY_ID_MESSAGE: &str = concat!(
             "invalid device key payload: keyId must be dk_hse_, dk_tpm_, or dk_osn_ ",