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/key_aliases.rs

@@ -0,0 +1,52 @@
+use toml::Value as TomlValue;
+use toml::map::Map as TomlMap;
+
+#[derive(Debug, Clone, Copy)]
+struct ConfigKeyAlias {
+    table_path: &'static [&'static str],
+    legacy_key: &'static str,
+    canonical_key: &'static str,
+}
+
+const CONFIG_KEY_ALIASES: &[ConfigKeyAlias] = &[ConfigKeyAlias {
+    table_path: &["memories"],
+    legacy_key: "no_memories_if_mcp_or_web_search",
+    canonical_key: "disable_on_external_context",
+}];
+
+pub(crate) fn normalize_key_aliases(path: &[String], table: &mut TomlMap<String, TomlValue>) {
+    for alias in CONFIG_KEY_ALIASES {
+        if path
+            .iter()
+            .map(String::as_str)
+            .eq(alias.table_path.iter().copied())
+            && let Some(value) = table.remove(alias.legacy_key)
+        {
+            table
+                .entry(alias.canonical_key.to_string())
+                .or_insert(value);
+        }
+    }
+}
+
+pub(crate) fn normalized_with_key_aliases(value: &TomlValue, path: &[String]) -> TomlValue {
+    match value {
+        TomlValue::Table(table) => {
+            let mut normalized = TomlMap::new();
+            for (key, child) in table {
+                let mut child_path = path.to_vec();
+                child_path.push(key.clone());
+                normalized.insert(key.clone(), normalized_with_key_aliases(child, &child_path));
+            }
+            normalize_key_aliases(path, &mut normalized);
+            TomlValue::Table(normalized)
+        }
+        TomlValue::Array(items) => TomlValue::Array(
+            items
+                .iter()
+                .map(|item| normalized_with_key_aliases(item, path))
+                .collect(),
+        ),
+        _ => value.clone(),
+    }
+}