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/remote_process.rs

@@ -0,0 +1,51 @@
+use async_trait::async_trait;
+use tokio::sync::broadcast;
+
+use crate::ExecProcess;
+use crate::ExecServerClient;
+use crate::ExecServerError;
+use crate::ExecServerEvent;
+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(Clone)]
+pub(crate) struct RemoteProcess {
+    client: ExecServerClient,
+}
+
+impl RemoteProcess {
+    pub(crate) fn new(client: ExecServerClient) -> Self {
+        Self { client }
+    }
+}
+
+#[async_trait]
+impl ExecProcess for RemoteProcess {
+    async fn start(&self, params: ExecParams) -> Result<ExecResponse, ExecServerError> {
+        self.client.exec(params).await
+    }
+
+    async fn read(&self, params: ReadParams) -> Result<ReadResponse, ExecServerError> {
+        self.client.read(params).await
+    }
+
+    async fn write(
+        &self,
+        process_id: &str,
+        chunk: Vec<u8>,
+    ) -> Result<WriteResponse, ExecServerError> {
+        self.client.write(process_id, chunk).await
+    }
+
+    async fn terminate(&self, process_id: &str) -> Result<TerminateResponse, ExecServerError> {
+        self.client.terminate(process_id).await
+    }
+
+    fn subscribe_events(&self) -> broadcast::Receiver<ExecServerEvent> {
+        self.client.event_receiver()
+    }
+}