send context window with task started (#2752)

- Send context window with task started
- Accounting for changing the model per turn

codex-rs/core/src/client.rs

@@ -35,6 +35,7 @@ use crate::flags::CODEX_RS_SSE_FIXTURE;
 use crate::model_family::ModelFamily;
 use crate::model_provider_info::ModelProviderInfo;
 use crate::model_provider_info::WireApi;
+use crate::openai_model_info::get_model_info;
 use crate::openai_tools::create_tools_json_for_responses_api;
 use crate::protocol::TokenUsage;
 use crate::user_agent::get_codex_user_agent;
@@ -90,6 +91,12 @@ impl ModelClient {
         }
     }
 
+    pub fn get_model_context_window(&self) -> Option<u64> {
+        self.config
+            .model_context_window
+            .or_else(|| get_model_info(&self.config.model_family).map(|info| info.context_window))
+    }
+
     /// Dispatches to either the Responses or Chat implementation depending on
     /// the provider config.  Public callers always invoke `stream()` – the
     /// specialised helpers are private to avoid accidental misuse.