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/common/test_codex.rs

@@ -11,7 +11,6 @@ use codex_core::ThreadManager;
 use codex_core::built_in_model_providers;
 use codex_core::config::Config;
 use codex_core::features::Feature;
-use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::protocol::AskForApproval;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::Op;
@@ -300,7 +299,7 @@ impl TestCodex {
                 sandbox_policy,
                 model: session_model,
                 effort: None,
-                summary: ReasoningSummary::Auto,
+                summary: None,
                 collaboration_mode: None,
                 personality: None,
             })