Allow clients not to send summary as an option (#12950)

Summary is a required parameter on UserTurn. Ideally we'd like the core
to decide the appropriate summary level.

Make the summary optional and don't send it when not needed.

codex-rs/core/tests/suite/skills.rs

@@ -77,7 +77,7 @@ async fn user_turn_includes_skill_instructions() -> Result<()> {
             sandbox_policy: SandboxPolicy::DangerFullAccess,
             model: session_model,
             effort: None,
-            summary: codex_protocol::config_types::ReasoningSummary::Auto,
+            summary: None,
             collaboration_mode: None,
             personality: None,
         })