Match Windows shell guidance to the executor platform (#41368)

## What changed

- Include Windows safety guidance in the `exec_command` tool description when a
  single ready environment reports Windows as its executor platform.
- Omit that guidance when the selected executor reports a non-Windows platform.
- Preserve the host-based behavior when platform information is unavailable or
  the tool can target multiple environments.

## Testing

- Cover Windows, non-Windows, unknown-platform, and multi-environment tool plans.
- Verify remote environments expose guidance that matches their target OS.

GitOrigin-RevId: 95e09790446c684ffaff2a0be077db426fff4618

codex-rs/core/tests/suite/remote_env.rs

@@ -327,7 +327,7 @@ async fn remote_test_env_can_connect_and_use_filesystem() -> Result<()> {
 }
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-async fn remote_test_env_exposes_target_shell_to_model() -> Result<()> {
+async fn remote_test_env_exposes_target_shell_and_exec_guidance_to_model() -> Result<()> {
     skip_if_no_remote_env!(Ok(()));
 
     let server = start_mock_server().await;
@@ -340,19 +340,26 @@ async fn remote_test_env_exposes_target_shell_to_model() -> Result<()> {
         ]),
     )
     .await;
-    let mut builder = test_codex().with_config(|config| {
-        config
-            .features
-            .disable(Feature::ShellTool)
-            .expect("test config should allow feature update");
-    });
+    let mut builder = test_codex();
     let test = builder.build_with_auto_env(&server).await?;
 
     test.submit_turn("report remote environment").await?;
 
     let request = response_mock.single_request();
-    let tools = tool_names(&request.body_json());
-    assert!(!tools.contains(&"exec_command".to_string()));
+    let body = request.body_json();
+    let exec_command = body["tools"]
+        .as_array()
+        .context("tools should be an array")?
+        .iter()
+        .find(|tool| tool["name"] == "exec_command")
+        .context("exec_command should be available")?;
+    let has_windows_guidance = exec_command["description"]
+        .as_str()
+        .is_some_and(|description| description.contains("Windows safety rules:"));
+    assert_eq!(
+        has_windows_guidance,
+        matches!(test_target_os(), TestTargetOs::Windows)
+    );
     let environment_context = request
         .message_input_texts("user")
         .into_iter()