[codex] Add unsandboxed process exec API (#19040)

## Why

App-server clients sometimes need argv-based local process execution
while sandbox policy is controlled outside Codex. Those environments can
reject sandbox-disabling paths before a command ever starts, even when
the caller intentionally wants unsandboxed execution.

This PR adds a distinct `process/*` API for that use case instead of
extending `command/exec` with another sandbox-disabling shape. Keeping
the new surface separate also makes the future removal of `command/exec`
simpler: clients that need explicit process lifecycle control can move
to the newer handle-based API without depending on `command/exec`
business logic.

## What changed

- Added v2 process lifecycle methods: `process/spawn`,
`process/writeStdin`, `process/resizePty`, and `process/kill`.
- Added process notifications: `process/outputDelta` for streamed
stdout/stderr chunks and `process/exited` for final exit status and
buffered output.
- Made `process/spawn` intentionally unsandboxed and omitted
sandbox-selection fields such as `sandboxPolicy` and
`permissionProfile`.
- Added client-supplied, connection-scoped `processHandle` values for
follow-up control requests and notification routing.
- Supported cwd, environment overrides, PTY mode and size, stdin
streaming, stdout/stderr streaming, per-stream output caps, and timeout
controls.
- Killed active process sessions when the originating app-server
connection closes.
- Wired the implementation through the modular `request_processors/`
app-server layout, with process-handle request serialization for
follow-up control calls.
- Updated generated JSON/TypeScript schema fixtures and documented the
new API in `codex-rs/app-server/README.md`.
- Added v2 app-server integration coverage in
`codex-rs/app-server/tests/suite/v2/process_exec.rs` for spawn
acknowledgement before exit, buffered output caps, and process
termination.

## Verification

- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-app-server`

---------

Co-authored-by: Owen Lin <owen@openai.com>

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

@@ -26,6 +26,7 @@ use crate::request_processors::InitializeRequestProcessor;
 use crate::request_processors::MarketplaceRequestProcessor;
 use crate::request_processors::McpRequestProcessor;
 use crate::request_processors::PluginRequestProcessor;
+use crate::request_processors::ProcessExecRequestProcessor;
 use crate::request_processors::SearchRequestProcessor;
 use crate::request_processors::ThreadGoalRequestProcessor;
 use crate::request_processors::ThreadRequestProcessor;
@@ -157,6 +158,7 @@ pub(crate) struct MessageProcessor {
     apps_processor: AppsRequestProcessor,
     catalog_processor: CatalogRequestProcessor,
     command_exec_processor: CommandExecRequestProcessor,
+    process_exec_processor: ProcessExecRequestProcessor,
     config_processor: ConfigRequestProcessor,
     device_key_processor: DeviceKeyRequestProcessor,
     external_agent_config_processor: ExternalAgentConfigRequestProcessor,
@@ -335,6 +337,7 @@ impl MessageProcessor {
             Arc::clone(&config),
             outgoing.clone(),
         );
+        let process_exec_processor = ProcessExecRequestProcessor::new(outgoing.clone());
         let feedback_processor = FeedbackRequestProcessor::new(
             auth_manager.clone(),
             Arc::clone(&thread_manager),
@@ -457,6 +460,7 @@ impl MessageProcessor {
             apps_processor,
             catalog_processor,
             command_exec_processor,
+            process_exec_processor,
             config_processor,
             device_key_processor,
             external_agent_config_processor,
@@ -675,6 +679,9 @@ impl MessageProcessor {
         self.command_exec_processor
             .connection_closed(connection_id)
             .await;
+        self.process_exec_processor
+            .connection_closed(connection_id)
+            .await;
         self.thread_processor.connection_closed(connection_id).await;
     }
 
@@ -1240,6 +1247,26 @@ impl MessageProcessor {
                     .command_exec_terminate(request_id.clone(), params)
                     .await
             }
+            ClientRequest::ProcessSpawn { params, .. } => self
+                .process_exec_processor
+                .process_spawn(request_id.clone(), params)
+                .await
+                .map(|()| None),
+            ClientRequest::ProcessWriteStdin { params, .. } => {
+                self.process_exec_processor
+                    .process_write_stdin(request_id.clone(), params)
+                    .await
+            }
+            ClientRequest::ProcessKill { params, .. } => {
+                self.process_exec_processor
+                    .process_kill(request_id.clone(), params)
+                    .await
+            }
+            ClientRequest::ProcessResizePty { params, .. } => {
+                self.process_exec_processor
+                    .process_resize_pty(request_id.clone(), params)
+                    .await
+            }
             ClientRequest::FeedbackUpload { params, .. } => {
                 self.feedback_processor.feedback_upload(params).await
             }