put workspace roots in the environment context (#3375)

to keep the tool description constant when the writable roots change.

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

@@ -426,11 +426,17 @@ async fn overrides_turn_context_but_keeps_cached_prefix_and_key_constant() {
     // After overriding the turn context, the environment context should be emitted again
     // reflecting the new approval policy and sandbox settings. Omit cwd because it did
     // not change.
-    let expected_env_text_2 = r#"<environment_context>
+    let expected_env_text_2 = format!(
+        r#"<environment_context>
   <approval_policy>never</approval_policy>
   <sandbox_mode>workspace-write</sandbox_mode>
   <network_access>enabled</network_access>
-</environment_context>"#;
+  <writable_roots>
+    <root>{}</root>
+  </writable_roots>
+</environment_context>"#,
+        writable.path().to_string_lossy()
+    );
     let expected_env_msg_2 = serde_json::json!({
         "type": "message",
         "role": "user",