add web_search_cached flag (#8795)

Add `web_search_cached` feature to config. Enables `web_search` tool
with access only to cached/indexed results (see
[docs](https://platform.openai.com/docs/guides/tools-web-search#live-internet-access)).

This takes precedence over the existing `web_search_request`, which
continues to enable `web_search` over live results as it did before.

`web_search_cached` is disabled for review mode, as `web_search_request`
is.

codex-rs/core/src/client_common.rs

@@ -195,8 +195,13 @@ pub(crate) mod tools {
         LocalShell {},
         // TODO: Understand why we get an error on web_search although the API docs say it's supported.
         // https://platform.openai.com/docs/guides/tools-web-search?api-mode=responses#:~:text=%7B%20type%3A%20%22web_search%22%20%7D%2C
+        // The `external_web_access` field determines whether the web search is over cached or live content.
+        // https://platform.openai.com/docs/guides/tools-web-search#live-internet-access
         #[serde(rename = "web_search")]
-        WebSearch {},
+        WebSearch {
+            #[serde(skip_serializing_if = "Option::is_none")]
+            external_web_access: Option<bool>,
+        },
         #[serde(rename = "custom")]
         Freeform(FreeformTool),
     }
@@ -206,7 +211,7 @@ pub(crate) mod tools {
             match self {
                 ToolSpec::Function(tool) => tool.name.as_str(),
                 ToolSpec::LocalShell {} => "local_shell",
-                ToolSpec::WebSearch {} => "web_search",
+                ToolSpec::WebSearch { .. } => "web_search",
                 ToolSpec::Freeform(tool) => tool.name.as_str(),
             }
         }