feat: config aliases (#18140)

Rename `no_memories_if_mcp_or_web_search` →
`disable_on_external_context` with backward compatibility

While doing so, we add a key alias system on our layer merging system.
What we try to avoid is a case where a company managed config use an old
name while the user has a new name in it's local config (which would
make the deserialization fail)

codex-rs/core/src/config/config_tests.rs

@@ -221,7 +221,7 @@ persistence = "none"
 
     let memories = r#"
 [memories]
-no_memories_if_mcp_or_web_search = true
+disable_on_external_context = true
 generate_memories = false
 use_memories = false
 max_raw_memories_for_consolidation = 512
@@ -236,7 +236,7 @@ consolidation_model = "gpt-5"
         toml::from_str::<ConfigToml>(memories).expect("TOML deserialization should succeed");
     assert_eq!(
         Some(MemoriesToml {
-            no_memories_if_mcp_or_web_search: Some(true),
+            disable_on_external_context: Some(true),
             generate_memories: Some(false),
             use_memories: Some(false),
             max_raw_memories_for_consolidation: Some(512),
@@ -260,7 +260,7 @@ consolidation_model = "gpt-5"
     assert_eq!(
         config.memories,
         MemoriesConfig {
-            no_memories_if_mcp_or_web_search: true,
+            disable_on_external_context: true,
             generate_memories: false,
             use_memories: false,
             max_raw_memories_for_consolidation: 512,
@@ -272,6 +272,18 @@ consolidation_model = "gpt-5"
             consolidation_model: Some("gpt-5".to_string()),
         }
     );
+
+    let legacy_memories_cfg =
+        toml::from_str::<ConfigToml>("[memories]\nno_memories_if_mcp_or_web_search = true\n")
+            .expect("legacy memories TOML should deserialize");
+    assert!(
+        MemoriesConfig::from(
+            legacy_memories_cfg
+                .memories
+                .expect("legacy memories config")
+        )
+        .disable_on_external_context
+    );
 }
 
 #[test]

codex-rs/core/src/mcp_tool_call.rs

@@ -541,11 +541,7 @@ async fn augment_mcp_tool_request_meta_with_sandbox_state(
 }
 
 async fn maybe_mark_thread_memory_mode_polluted(sess: &Session, turn_context: &TurnContext) {
-    if !turn_context
-        .config
-        .memories
-        .no_memories_if_mcp_or_web_search
-    {
+    if !turn_context.config.memories.disable_on_external_context {
         return;
     }
     state_db::mark_thread_memory_mode_polluted(

codex-rs/core/src/stream_events_utils.rs

@@ -154,10 +154,7 @@ pub(crate) async fn mark_thread_memory_mode_polluted_if_external_context(
     turn_context: &TurnContext,
     item: &ResponseItem,
 ) {
-    if !turn_context
-        .config
-        .memories
-        .no_memories_if_mcp_or_web_search
+    if !turn_context.config.memories.disable_on_external_context
         || !response_item_may_include_external_context(item)
     {
         return;