Enable the Bedrock setup wizard by default (#56033)

## What changed

Promote `bedrock_setup_wizard` to stable and enable it by default for eligible onboarding. Users can still disable it with `features.bedrock_setup_wizard = false`.

## Testing

Update onboarding coverage to check the enabled default and explicit opt-out. Remove the explicit opt-in from daemon startup tests so they exercise the default configuration.

GitOrigin-RevId: 5bd911088681a31d34aa444398e6d3f7d1d87955

codex-rs/cli/tests/daemon_startup.rs

@@ -179,7 +179,7 @@ async fn daemon_startup(command: &str) -> Result<()> {
         home.path().join("config.toml"),
         format!(
             "model = \"gpt-5.6-terra\"\n\
-         features.bedrock_setup_wizard = true\ncli_auth_credentials_store = \"file\"\n\
+         cli_auth_credentials_store = \"file\"\n\
          suppress_unstable_features_warning = true\nanalytics.enabled = false\n\
          windows.sandbox = \"unelevated\"\n\
          tui.disable_paste_burst = true\n\