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_common.rs

@@ -93,6 +93,10 @@ pub enum ResponseEvent {
     ReasoningSummaryDelta(String),
     ReasoningContentDelta(String),
     ReasoningSummaryPartAdded,
+    WebSearchCallBegin {
+        call_id: String,
+        query: Option<String>,
+    },
 }
 
 #[derive(Debug, Serialize)]