[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/core/src/connectors.rs

@@ -601,6 +601,11 @@ pub(crate) fn mcp_approvals_reviewer(
             connector_id
                 .and_then(|connector_id| apps_config.apps.get(connector_id))
                 .and_then(|app| app.approvals_reviewer)
+                .or_else(|| {
+                    apps_config
+                        .default
+                        .and_then(|defaults| defaults.approvals_reviewer)
+                })
         })
     } else {
         None