Expose structured standalone web search results (#32898)

## Why

Standalone web search can return structured result DTOs separately from its
model-facing text output. App-server clients need access to that data without
coupling Codex to every result type or field.

## What changed

- Preserve the optional `results` array from `/v1/alpha/search` as opaque JSON
  through web-search completion events, thread history, and app-server
  `webSearch` items.
- Keep responses from older endpoints compatible when `results` is absent, and
  allow clients to ignore result types and fields they do not understand.
- Regenerate the protocol schemas and TypeScript definitions for the optional
  field.

## Testing

- Cover responses with missing, empty, and forward-compatible result payloads.
- Verify standalone web-search results round-trip through app-server item
  notifications and thread-item conversion.

GitOrigin-RevId: 739ef5694ece5733f7b8749ec41d6bfa3f33c8c5

codex-rs/ext/items/src/tests.rs

@@ -46,6 +46,7 @@ fn web_search_item_preserves_stable_wire_shape() {
             query: Some("docs".to_string()),
             queries: None,
         }),
+        results: None,
     });
     let value = serde_json::to_value(&item).expect("serialize extension item");
 
@@ -60,12 +61,27 @@ fn web_search_item_preserves_stable_wire_shape() {
                 "query": "docs",
                 "queries": null,
             },
+            "results": null,
         })
     );
     assert_eq!(
         serde_json::from_value::<ExtensionItem>(value).expect("deserialize extension item"),
         item
     );
+    assert_eq!(
+        serde_json::from_value::<ExtensionItem>(json!({
+            "kind": "web.search",
+            "id": "search-1",
+            "query": "docs",
+            "action": {
+                "type": "search",
+                "query": "docs",
+                "queries": null,
+            },
+        }))
+        .expect("deserialize legacy extension item without results"),
+        item
+    );
 }
 
 #[test]