feat(app-server): add an --analytics-default-enabled flag (#9118)

Add a new `codex app-server --analytics-default-enabled` CLI flag that
controls whether analytics are enabled by default.

Analytics are disabled by default for app-server. Users have to
explicitly opt in
via the `analytics` section in the config.toml file.

However, for first-party use cases like the VSCode IDE extension, we
default analytics
to be enabled by default by setting this flag. Users can still opt out
by setting this
in their config.toml:

```toml
[analytics]
enabled = false
```

See https://developers.openai.com/codex/config-advanced/#metrics for
more details.

codex-rs/app-server/tests/suite/v2/mod.rs

@@ -1,4 +1,5 @@
 mod account;
+mod analytics;
 mod config_rpc;
 mod initialize;
 mod model_list;