Support packaged defaults in config layering (#37810)

## What changed

- Load an optional package-supplied config file as the lowest-precedence
  configuration layer, so every existing layer can override its values.
- Report the layer as `packagedDefaults`, including its source path, through
  config diagnostics and the app-server protocol.
- Return an error when a configured packaged defaults file is missing.

## Testing

- Cover precedence across packaged, system, user, and session layers.
- Cover the missing-file error path.

GitOrigin-RevId: 52406890f4d580eedabc90bf2a9ee3affa36fee1

codex-rs/app-server-protocol/src/protocol/v2/config.rs

@@ -27,6 +27,14 @@ use std::path::PathBuf;
 #[ts(tag = "type")]
 #[ts(export_to = "v2/")]
 pub enum ConfigLayerSource {
+    /// Default configuration supplied with the installed Codex package.
+    #[serde(rename_all = "camelCase")]
+    #[ts(rename_all = "camelCase")]
+    PackagedDefaults {
+        /// Path to the packaged default configuration file.
+        file: AbsolutePathBuf,
+    },
+
     /// Managed preferences layer delivered by MDM (macOS only).
     #[serde(rename_all = "camelCase")]
     #[ts(rename_all = "camelCase")]
@@ -102,6 +110,7 @@ impl ConfigLayerSource {
     /// from a layer with a lower precedence.
     pub fn precedence(&self) -> i16 {
         match self {
+            ConfigLayerSource::PackagedDefaults { .. } => -10,
             ConfigLayerSource::Mdm { .. } => 0,
             ConfigLayerSource::System { .. } => 10,
             ConfigLayerSource::EnterpriseManaged { .. } => 15,