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/schema/json/codex_app_server_protocol.schemas.json

@@ -8509,6 +8509,32 @@
       },
       "ConfigLayerSource": {
         "oneOf": [
+          {
+            "description": "Default configuration supplied with the installed Codex package.",
+            "properties": {
+              "file": {
+                "allOf": [
+                  {
+                    "$ref": "#/definitions/v2/AbsolutePathBuf"
+                  }
+                ],
+                "description": "Path to the packaged default configuration file."
+              },
+              "type": {
+                "enum": [
+                  "packagedDefaults"
+                ],
+                "title": "PackagedDefaultsConfigLayerSourceType",
+                "type": "string"
+              }
+            },
+            "required": [
+              "file",
+              "type"
+            ],
+            "title": "PackagedDefaultsConfigLayerSource",
+            "type": "object"
+          },
           {
             "description": "Managed preferences layer delivered by MDM (macOS only).",
             "properties": {