Add exec-server exec RPC implementation (#15090)

Stacked PR 2/3, based on the stub PR.

Adds the exec RPC implementation and process/event flow in exec-server
only.

---------

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

codex-rs/exec-server/src/server/handler/tests.rs

@@ -0,0 +1,102 @@
+use std::collections::HashMap;
+use std::sync::Arc;
+use std::time::Duration;
+
+use pretty_assertions::assert_eq;
+use tokio::sync::mpsc;
+
+use super::ExecServerHandler;
+use crate::protocol::ExecParams;
+use crate::protocol::InitializeResponse;
+use crate::protocol::TerminateParams;
+use crate::protocol::TerminateResponse;
+use crate::rpc::RpcNotificationSender;
+
+fn exec_params(process_id: &str) -> ExecParams {
+    let mut env = HashMap::new();
+    if let Some(path) = std::env::var_os("PATH") {
+        env.insert("PATH".to_string(), path.to_string_lossy().into_owned());
+    }
+    ExecParams {
+        process_id: process_id.to_string(),
+        argv: vec![
+            "bash".to_string(),
+            "-lc".to_string(),
+            "sleep 0.1".to_string(),
+        ],
+        cwd: std::env::current_dir().expect("cwd"),
+        env,
+        tty: false,
+        arg0: None,
+    }
+}
+
+async fn initialized_handler() -> Arc<ExecServerHandler> {
+    let (outgoing_tx, _outgoing_rx) = mpsc::channel(16);
+    let handler = Arc::new(ExecServerHandler::new(RpcNotificationSender::new(
+        outgoing_tx,
+    )));
+    assert_eq!(
+        handler.initialize().expect("initialize"),
+        InitializeResponse {}
+    );
+    handler.initialized().expect("initialized");
+    handler
+}
+
+#[tokio::test]
+async fn duplicate_process_ids_allow_only_one_successful_start() {
+    let handler = initialized_handler().await;
+    let first_handler = Arc::clone(&handler);
+    let second_handler = Arc::clone(&handler);
+
+    let (first, second) = tokio::join!(
+        first_handler.exec(exec_params("proc-1")),
+        second_handler.exec(exec_params("proc-1")),
+    );
+
+    let (successes, failures): (Vec<_>, Vec<_>) =
+        [first, second].into_iter().partition(Result::is_ok);
+    assert_eq!(successes.len(), 1);
+    assert_eq!(failures.len(), 1);
+
+    let error = failures
+        .into_iter()
+        .next()
+        .expect("one failed request")
+        .expect_err("expected duplicate process error");
+    assert_eq!(error.code, -32600);
+    assert_eq!(error.message, "process proc-1 already exists");
+
+    tokio::time::sleep(Duration::from_millis(150)).await;
+    handler.shutdown().await;
+}
+
+#[tokio::test]
+async fn terminate_reports_false_after_process_exit() {
+    let handler = initialized_handler().await;
+    handler
+        .exec(exec_params("proc-1"))
+        .await
+        .expect("start process");
+
+    let deadline = tokio::time::Instant::now() + Duration::from_secs(1);
+    loop {
+        let response = handler
+            .terminate(TerminateParams {
+                process_id: "proc-1".to_string(),
+            })
+            .await
+            .expect("terminate response");
+        if response == (TerminateResponse { running: false }) {
+            break;
+        }
+        assert!(
+            tokio::time::Instant::now() < deadline,
+            "process should have exited within 1s"
+        );
+        tokio::time::sleep(Duration::from_millis(25)).await;
+    }
+
+    handler.shutdown().await;
+}