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/tool_parallelism.rs

@@ -5,7 +5,6 @@ use std::fs;
 use std::time::Duration;
 use std::time::Instant;
 
-use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::protocol::AskForApproval;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::Op;
@@ -46,7 +45,7 @@ async fn run_turn(test: &TestCodex, prompt: &str) -> anyhow::Result<()> {
             sandbox_policy: SandboxPolicy::DangerFullAccess,
             model: session_model,
             effort: None,
-            summary: ReasoningSummary::Auto,
+            summary: None,
             collaboration_mode: None,
             personality: None,
         })
@@ -361,7 +360,7 @@ async fn shell_tools_start_before_response_completed_when_stream_delayed() -> an
             sandbox_policy: SandboxPolicy::DangerFullAccess,
             model: session_model,
             effort: None,
-            summary: ReasoningSummary::Auto,
+            summary: None,
             collaboration_mode: None,
             personality: None,
         })