feat: polluted memories (#13008)

Add a feature flag to disable memory creation for "polluted"

codex-rs/protocol/src/protocol.rs

@@ -2060,6 +2060,8 @@ pub struct SessionMeta {
     pub base_instructions: Option<BaseInstructions>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub dynamic_tools: Option<Vec<DynamicToolSpec>>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub memory_mode: Option<String>,
 }
 
 impl Default for SessionMeta {
@@ -2077,6 +2079,7 @@ impl Default for SessionMeta {
             model_provider: None,
             base_instructions: None,
             dynamic_tools: None,
+            memory_mode: None,
         }
     }
 }