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/merge_tests.rs

@@ -0,0 +1,100 @@
+use super::*;
+use crate::config_toml::ConfigToml;
+use crate::types::MemoriesToml;
+use pretty_assertions::assert_eq;
+
+fn parse_toml(value: &str) -> TomlValue {
+    toml::from_str(value).expect("TOML should parse")
+}
+
+#[test]
+fn merge_toml_values_normalizes_legacy_key_from_base_layer() {
+    let mut base = parse_toml(
+        r#"
+[memories]
+no_memories_if_mcp_or_web_search = false
+"#,
+    );
+    let overlay = parse_toml(
+        r#"
+[memories]
+disable_on_external_context = true
+"#,
+    );
+
+    merge_toml_values(&mut base, &overlay);
+
+    let expected = parse_toml(
+        r#"
+[memories]
+disable_on_external_context = true
+"#,
+    );
+    assert_eq!(base, expected);
+
+    let config: ConfigToml = base.try_into().expect("merged config should deserialize");
+    assert_eq!(
+        config.memories,
+        Some(MemoriesToml {
+            disable_on_external_context: Some(true),
+            ..Default::default()
+        })
+    );
+}
+
+#[test]
+fn merge_toml_values_normalizes_legacy_key_from_overlay_layer() {
+    let mut base = parse_toml(
+        r#"
+[memories]
+disable_on_external_context = false
+"#,
+    );
+    let overlay = parse_toml(
+        r#"
+[memories]
+no_memories_if_mcp_or_web_search = true
+"#,
+    );
+
+    merge_toml_values(&mut base, &overlay);
+
+    let expected = parse_toml(
+        r#"
+[memories]
+disable_on_external_context = true
+"#,
+    );
+    assert_eq!(base, expected);
+
+    let config: ConfigToml = base.try_into().expect("merged config should deserialize");
+    assert_eq!(
+        config.memories,
+        Some(MemoriesToml {
+            disable_on_external_context: Some(true),
+            ..Default::default()
+        })
+    );
+}
+
+#[test]
+fn merge_toml_values_prefers_canonical_key_when_one_layer_has_both_names() {
+    let mut base = TomlValue::Table(toml::map::Map::new());
+    let overlay = parse_toml(
+        r#"
+[memories]
+disable_on_external_context = true
+no_memories_if_mcp_or_web_search = false
+"#,
+    );
+
+    merge_toml_values(&mut base, &overlay);
+
+    let expected = parse_toml(
+        r#"
+[memories]
+disable_on_external_context = true
+"#,
+    );
+    assert_eq!(base, expected);
+}