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/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;