Split exec process into local and remote implementations (#15233)

## Summary
- match the exec-process structure to filesystem PR #15232
- expose `ExecProcess` on `Environment`
- make `LocalProcess` the real implementation and `RemoteProcess` a thin
network proxy over `ExecServerClient`
- make `ProcessHandler` a thin RPC adapter delegating to `LocalProcess`
- add a shared local/remote process test

## Validation
- `just fmt`
- `CARGO_TARGET_DIR=~/.cache/cargo-target/codex cargo test -p
codex-exec-server`
- `just fix -p codex-exec-server`

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/exec-server/src/process.rs

@@ -0,0 +1,35 @@
+use async_trait::async_trait;
+use tokio::sync::broadcast;
+
+use crate::ExecServerError;
+use crate::protocol::ExecExitedNotification;
+use crate::protocol::ExecOutputDeltaNotification;
+use crate::protocol::ExecParams;
+use crate::protocol::ExecResponse;
+use crate::protocol::ReadParams;
+use crate::protocol::ReadResponse;
+use crate::protocol::TerminateResponse;
+use crate::protocol::WriteResponse;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum ExecServerEvent {
+    OutputDelta(ExecOutputDeltaNotification),
+    Exited(ExecExitedNotification),
+}
+
+#[async_trait]
+pub trait ExecProcess: Send + Sync {
+    async fn start(&self, params: ExecParams) -> Result<ExecResponse, ExecServerError>;
+
+    async fn read(&self, params: ReadParams) -> Result<ReadResponse, ExecServerError>;
+
+    async fn write(
+        &self,
+        process_id: &str,
+        chunk: Vec<u8>,
+    ) -> Result<WriteResponse, ExecServerError>;
+
+    async fn terminate(&self, process_id: &str) -> Result<TerminateResponse, ExecServerError>;
+
+    fn subscribe_events(&self) -> broadcast::Receiver<ExecServerEvent>;
+}