[ez][codex-rs] Support approvals reviewer in app defaults (#27075)

[from codex]

## Summary

- add `approvals_reviewer` support to `[apps._default]`
- resolve connected-app reviewers in per-app, app-default, then global
order
- expose the setting through the v2 config API and regenerate schema
fixtures

## Context

PR #25167 added `apps.<connector_id>.approvals_reviewer`, but the shared
app defaults table could not specify the reviewer. This extends the same
behavior to `[apps._default]` while preserving per-app overrides.

Managed `allowed_approvals_reviewers` requirements still constrain both
default and per-app values. A disallowed app value falls back to the
global reviewer, and non-app MCP servers continue using the global
reviewer.

## Testing

- `just write-config-schema`
- `just write-app-server-schema`
- `just fmt`
- `just test -p codex-config`
- `just test -p codex-core app_approvals_reviewer`
- `just test -p codex-app-server-protocol`
- `just test -p codex-app-server config_read_includes_apps`

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -6584,6 +6584,16 @@
       },
       "AppsDefaultConfig": {
         "properties": {
+          "approvals_reviewer": {
+            "anyOf": [
+              {
+                "$ref": "#/definitions/v2/ApprovalsReviewer"
+              },
+              {
+                "type": "null"
+              }
+            ]
+          },
           "destructive_enabled": {
             "default": true,
             "type": "boolean"