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/app-server-protocol/schema/json/v2/ThreadMetadataUpdateResponse.json

@@ -1566,6 +1566,15 @@
             "query": {
               "type": "string"
             },
+            "results": {
+              "default": null,
+              "description": "Structured search results returned out-of-band by standalone web search.\n\nThese stay as opaque JSON at the extension/app-server boundary so new result fields and result types can pass through without a Codex release.",
+              "items": true,
+              "type": [
+                "array",
+                "null"
+              ]
+            },
             "type": {
               "enum": [
                 "webSearch"