Send prompt_cache_key (#2200)

To optimize prompt caching performance.

codex-rs/core/src/client.rs

@@ -169,6 +169,7 @@ impl ModelClient {
             store,
             stream: true,
             include,
+            prompt_cache_key: Some(self.session_id.to_string()),
         };
 
         let mut attempt = 0;

codex-rs/core/src/client_common.rs

@@ -215,6 +215,8 @@ pub(crate) struct ResponsesApiRequest<'a> {
     pub(crate) store: bool,
     pub(crate) stream: bool,
     pub(crate) include: Vec<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub(crate) prompt_cache_key: Option<String>,
 }
 
 pub(crate) fn create_reasoning_param_for_request(