feat: fork pattern v2 (#15771)

Adds this:
```
properties.insert(
            "fork_turns".to_string(),
            JsonSchema::String {
                description: Some(
                    "Optional MultiAgentV2 fork mode. Use `none`, `all`, or a positive integer string such as `3` to fork only the most recent turns."
                        .to_string(),
                ),
            },
        );
        ```

---------

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

codex-rs/tools/src/agent_tool_tests.rs

@@ -56,6 +56,8 @@ fn spawn_agent_tool_v2_requires_task_name_and_lists_visible_models() {
     assert!(description.contains("visible display (`visible-model`)"));
     assert!(!description.contains("hidden display (`hidden-model`)"));
     assert!(properties.contains_key("task_name"));
+    assert!(properties.contains_key("fork_turns"));
+    assert!(!properties.contains_key("fork_context"));
     assert_eq!(
         properties.get("agent_type"),
         Some(&JsonSchema::String {
@@ -69,6 +71,24 @@ fn spawn_agent_tool_v2_requires_task_name_and_lists_visible_models() {
     );
 }
 
+#[test]
+fn spawn_agent_tool_v1_keeps_legacy_fork_context_field() {
+    let tool = create_spawn_agent_tool_v1(SpawnAgentToolOptions {
+        available_models: &[],
+        agent_type_description: "role help".to_string(),
+    });
+
+    let ToolSpec::Function(ResponsesApiTool { parameters, .. }) = tool else {
+        panic!("spawn_agent should be a function tool");
+    };
+    let JsonSchema::Object { properties, .. } = parameters else {
+        panic!("spawn_agent should use object params");
+    };
+
+    assert!(properties.contains_key("fork_context"));
+    assert!(!properties.contains_key("fork_turns"));
+}
+
 #[test]
 fn send_message_tool_requires_items_and_uses_submission_output() {
     let ToolSpec::Function(ResponsesApiTool {