Session-level model client (#10664)

Make ModelClient a session-scoped object.
Move state that is session level onto the client, and make state that is
per-turn explicit on corresponding methods.
Stop taking a huge Config object, instead only pass in values that are
actually needed.

---------

Co-authored-by: Josh McKinney <joshka@openai.com>

codex-rs/core/src/compact_remote.rs

@@ -76,9 +76,14 @@ async fn run_remote_compact_task_inner_impl(
         output_schema: None,
     };
 
-    let mut new_history = turn_context
-        .client
-        .compact_conversation_history(&prompt)
+    let mut new_history = sess
+        .services
+        .model_client
+        .compact_conversation_history(
+            &prompt,
+            &turn_context.model_info,
+            &turn_context.otel_manager,
+        )
         .await?;
 
     if !ghost_snapshots.is_empty() {