Add web search tool (#2371)

Adds web_search tool, enabling the model to use Responses API web_search
tool.
- Disabled by default, enabled by --search flag
- When --search is passed, exposes web_search_request function tool to
the model, which triggers user approval. When approved, the model can
use the web_search tool for the remainder of the turn
<img width="1033" height="294" alt="image"
src="https://github.com/user-attachments/assets/62ac6563-b946-465c-ba5d-9325af28b28f"
/>

---------

Co-authored-by: easong-openai <easong@openai.com>

codex-rs/core/src/client.rs

@@ -149,7 +149,21 @@ impl ModelClient {
         let store = prompt.store && auth_mode != Some(AuthMode::ChatGPT);
 
         let full_instructions = prompt.get_full_instructions(&self.config.model_family);
-        let tools_json = create_tools_json_for_responses_api(&prompt.tools)?;
+        let mut tools_json = create_tools_json_for_responses_api(&prompt.tools)?;
+        // ChatGPT backend expects the preview name for web search.
+        if auth_mode == Some(AuthMode::ChatGPT) {
+            for tool in &mut tools_json {
+                if let Some(map) = tool.as_object_mut()
+                    && map.get("type").and_then(|v| v.as_str()) == Some("web_search")
+                {
+                    map.insert(
+                        "type".to_string(),
+                        serde_json::Value::String("web_search_preview".to_string()),
+                    );
+                }
+            }
+        }
+
         let reasoning = create_reasoning_param_for_request(
             &self.config.model_family,
             self.effort,
@@ -466,7 +480,8 @@ async fn process_sse<S>(
             }
         };
 
-        trace!("SSE event: {}", sse.data);
+        let raw = sse.data.clone();
+        trace!("SSE event: {}", raw);
 
         let event: SseEvent = match serde_json::from_str(&sse.data) {
             Ok(event) => event,
@@ -580,8 +595,24 @@ async fn process_sse<S>(
             | "response.in_progress"
             | "response.output_item.added"
             | "response.output_text.done" => {
-                // Currently, we ignore this event, but we handle it
-                // separately to skip the logging message in the `other` case.
+                if event.kind == "response.output_item.added"
+                    && let Some(item) = event.item.as_ref()
+                {
+                    // Detect web_search_call begin and forward a synthetic event upstream.
+                    if let Some(ty) = item.get("type").and_then(|v| v.as_str())
+                        && ty == "web_search_call"
+                    {
+                        let call_id = item
+                            .get("id")
+                            .and_then(|v| v.as_str())
+                            .unwrap_or("")
+                            .to_string();
+                        let ev = ResponseEvent::WebSearchCallBegin { call_id, query: None };
+                        if tx_event.send(Ok(ev)).await.is_err() {
+                            return;
+                        }
+                    }
+                }
             }
             "response.reasoning_summary_part.added" => {
                 // Boundary between reasoning summary sections (e.g., titles).
@@ -591,7 +622,7 @@ async fn process_sse<S>(
                 }
             }
             "response.reasoning_summary_text.done" => {}
-            other => debug!(other, "sse event"),
+            _ => {}
         }
     }
 }