fix: handle JSON Schema in additionalProperties for MCP tools (#4454)

Fixes #4176

Some common tools provide a schema (even if just an empty object schema)
as the value for `additionalProperties`. The parsing as it currently
stands fails when it encounters this. This PR updates the schema to
accept a schema object in addition to a boolean value, per the JSON
Schema spec.

codex-rs/core/src/plan_tool.rs

@@ -32,7 +32,7 @@ pub(crate) static PLAN_TOOL: LazyLock<OpenAiTool> = LazyLock::new(|| {
         items: Box::new(JsonSchema::Object {
             properties: plan_item_props,
             required: Some(vec!["step".to_string(), "status".to_string()]),
-            additional_properties: Some(false),
+            additional_properties: Some(false.into()),
         }),
     };
 
@@ -54,7 +54,7 @@ At most one step can be in_progress at a time.
         parameters: JsonSchema::Object {
             properties,
             required: Some(vec!["plan".to_string()]),
-            additional_properties: Some(false),
+            additional_properties: Some(false.into()),
         },
     })
 });