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/exec/tests/event_processor_with_json_output.rs

@@ -370,6 +370,7 @@ fn web_search_completion_preserves_query_and_action() {
                     query: Some("rust async await".to_string()),
                     queries: None,
                 }),
+                results: None,
             }),
             thread_id: "thread-1".to_string(),
             turn_id: "turn-1".to_string(),
@@ -408,6 +409,7 @@ fn web_search_start_and_completion_reuse_item_id() {
                 id: "search-1".to_string(),
                 query: String::new(),
                 action: None,
+                results: None,
             }),
             thread_id: "thread-1".to_string(),
             turn_id: "turn-1".to_string(),
@@ -423,6 +425,7 @@ fn web_search_start_and_completion_reuse_item_id() {
                     query: Some("rust async await".to_string()),
                     queries: None,
                 }),
+                results: None,
             }),
             thread_id: "thread-1".to_string(),
             turn_id: "turn-1".to_string(),