Support deferred loading for freeform tools (#36856)

## What changed

- Add optional `defer_loading` support to freeform Responses API tool definitions.
- Omit the field when it is unset so existing eager tool definitions retain their wire shape.

## Testing

- Verify legacy freeform tool deserialization and eager and deferred serialization shapes.

GitOrigin-RevId: 50e8658a54ac5b6ae0c1dbfe65f7bb62efef561d

codex-rs/tools/src/code_mode_tests.rs

@@ -71,6 +71,7 @@ fn augment_tool_spec_for_code_mode_preserves_exec_tool_description() {
         augment_tool_spec_for_code_mode(ToolSpec::Freeform(FreeformTool {
             name: codex_code_mode::PUBLIC_TOOL_NAME.to_string(),
             description: "Run code".to_string(),
+            defer_loading: None,
             format: FreeformToolFormat {
                 r#type: "grammar".to_string(),
                 syntax: "lark".to_string(),
@@ -80,6 +81,7 @@ fn augment_tool_spec_for_code_mode_preserves_exec_tool_description() {
         ToolSpec::Freeform(FreeformTool {
             name: codex_code_mode::PUBLIC_TOOL_NAME.to_string(),
             description: "Run code".to_string(),
+            defer_loading: None,
             format: FreeformToolFormat {
                 r#type: "grammar".to_string(),
                 syntax: "lark".to_string(),
@@ -94,6 +96,7 @@ fn tool_spec_to_code_mode_tool_definition_returns_augmented_nested_tools() {
     let spec = ToolSpec::Freeform(FreeformTool {
         name: "apply_patch".to_string(),
         description: "Apply a patch".to_string(),
+        defer_loading: None,
         format: FreeformToolFormat {
             r#type: "grammar".to_string(),
             syntax: "lark".to_string(),