Support nested canonical paths in config key aliases (#47904)

## What changed

Represent legacy and canonical config keys as full paths, allowing aliases to target nested tables within the legacy key's containing table. Create missing destination tables, preserve existing canonical values, and skip insertion when an intermediate value is not a table.

Keep the existing `memories` and `agents` alias mappings unchanged.

GitOrigin-RevId: 9ef17e9d1b6adf6299777ed0051cb301fd245073

codex-rs/config/src/key_aliases.rs

@@ -5,35 +5,49 @@ 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,
+    legacy: &'static [&'static str],
+    canonical: &'static [&'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",
+        legacy: &["memories", "no_memories_if_mcp_or_web_search"],
+        canonical: &["memories", "disable_on_external_context"],
     },
     ConfigKeyAlias {
-        table_path: &["agents"],
-        legacy_key: "max_threads",
-        canonical_key: "max_concurrent_threads_per_session",
+        legacy: &["agents", "max_threads"],
+        canonical: &["agents", "max_concurrent_threads_per_session"],
     },
 ];
 
 fn normalize_table_key_aliases(path: &[String], table: &mut TomlMap<String, TomlValue>) {
-    for alias in CONFIG_KEY_ALIASES {
+    'aliases: for alias in CONFIG_KEY_ALIASES {
+        // Destinations stay within the legacy key's containing table.
+        let Some((legacy_key, table_path)) = alias.legacy.split_last() else {
+            continue;
+        };
         if path
             .iter()
             .map(String::as_str)
-            .eq(alias.table_path.iter().copied())
-            && let Some(value) = table.remove(alias.legacy_key)
+            .eq(table_path.iter().copied())
+            && let Some(canonical_path) = alias.canonical.strip_prefix(table_path)
+            && let Some(value) = table.remove(*legacy_key)
         {
-            table
-                .entry(alias.canonical_key.to_string())
-                .or_insert(value);
+            // Insert at the canonical path if absent.
+            let Some((key, parents)) = canonical_path.split_last() else {
+                continue;
+            };
+            let mut destination = &mut *table;
+            for parent in parents {
+                let child = destination
+                    .entry((*parent).to_string())
+                    .or_insert_with(|| TomlValue::Table(TomlMap::new()));
+                let Some(child) = child.as_table_mut() else {
+                    continue 'aliases;
+                };
+                destination = child;
+            }
+            destination.entry((*key).to_string()).or_insert(value);
         }
     }
 }