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/config/src/state_tests.rs

@@ -0,0 +1,34 @@
+use super::*;
+use pretty_assertions::assert_eq;
+
+#[test]
+fn origins_use_canonical_key_aliases() {
+    let layer = ConfigLayerEntry::new(
+        ConfigLayerSource::SessionFlags,
+        toml::from_str(
+            r#"
+[memories]
+no_memories_if_mcp_or_web_search = true
+"#,
+        )
+        .expect("config TOML should parse"),
+    );
+    let metadata = layer.metadata();
+    let stack = ConfigLayerStack::new(
+        vec![layer],
+        ConfigRequirements::default(),
+        ConfigRequirementsToml::default(),
+    )
+    .expect("single layer stack should be valid");
+
+    let origins = stack.origins();
+
+    assert_eq!(
+        origins.get("memories.disable_on_external_context"),
+        Some(&metadata)
+    );
+    assert!(
+        !origins.contains_key("memories.no_memories_if_mcp_or_web_search"),
+        "legacy key should be canonicalized before origin recording"
+    );
+}