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/truncation.rs

@@ -5,7 +5,6 @@ use anyhow::Context;
 use anyhow::Result;
 use codex_core::config::types::McpServerConfig;
 use codex_core::config::types::McpServerTransportConfig;
-use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::protocol::AskForApproval;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::Op;
@@ -488,7 +487,7 @@ async fn mcp_image_output_preserves_image_and_no_text_summary() -> Result<()> {
             sandbox_policy: SandboxPolicy::new_read_only_policy(),
             model: session_model,
             effort: None,
-            summary: ReasoningSummary::Auto,
+            summary: None,
             collaboration_mode: None,
             personality: None,
         })