Support anyOf and enum in JsonSchema (#16875)

This brings us into better alignment with the JSON schema subset that is
supported in
<https://developers.openai.com/api/docs/guides/structured-outputs#supported-schemas>,
and also allows us to render richer function signatures in code mode
(e.g., anyOf{null, OtherObjectType})

codex-rs/tools/src/js_repl_tool_tests.rs

@@ -1,4 +1,5 @@
 use super::*;
+use crate::JsonSchema;
 use crate::ToolSpec;
 use pretty_assertions::assert_eq;
 use std::collections::BTreeMap;
@@ -29,11 +30,11 @@ fn js_repl_reset_tool_matches_expected_spec() {
                     .to_string(),
             strict: false,
             defer_loading: None,
-            parameters: JsonSchema::Object {
-                properties: BTreeMap::new(),
-                required: None,
-                additional_properties: Some(false.into()),
-            },
+            parameters: JsonSchema::object(
+                BTreeMap::new(),
+                /*required*/ None,
+                Some(false.into())
+            ),
             output_schema: None,
         })
     );