Playground latest runtime fails before WordPress boot with SQLite ABSPATH fatal

## Summary
Running a WP Codebox recipe with the default `latest` WordPress runtime can fail before WordPress boots. The failure happens inside Playground's SQLite preload path, before caller code or recipe workflow steps execute.

## Observed failure
A Studio Web website-generation bench run using WP Codebox recipe-run failed during `workflow.steps[0]` startup with:

```text
Recipe workflow steps[0] failed: WordPress Playground CLI exited while booting the runtime with exit code 1.
Fatal error: Uncaught Error: Undefined constant "ABSPATH" in /internal/shared/sqlite-database-integration/constants.php:39
Stack trace:
#0 /internal/shared/sqlite-database-integration/wp-includes/sqlite/db.php(15): require_once()
#1 /internal/shared/mu-plugins/sqlite-database-integration.php(42): require_once('/internal/share...')
#2 /internal/shared/preload/0-sqlite.php(57): require_once('/internal/share...')
#3 /internal/shared/preload/0-sqlite.php(33): Playground_SQLite_Integration_Loader->load_sqlite_integration()
#4 /internal/eval.php(10): Playground_SQLite_Integration_Loader->__call('check_connectio...', Array)
```

## Expected behavior
`latest` should be a valid default WordPress runtime for WP Codebox recipes, including recipes that need the current WordPress AI/client runtime surface.

## Notes
- This is separate from PR #715, which centralizes WP Codebox's default WordPress runtime as `latest`.
- Do not work around this by downgrading the default runtime. Downstream callers can explicitly pin a version when they intentionally need an older runtime, but the default should remain current.

## AI assistance
- **AI assistance:** Yes
- **Tool(s):** OpenCode (GPT-5.5)
- **Used for:** Capturing the observed failure and drafting the issue from lab run output.

Inizia riproducendo un'esecuzione di recipe di WP Codebox con il runtime latest predefinito e analizza l'avvio di workflow.steps[0]. Segui il percorso di precaricamento SQLite mostrato nell'errore, in particolare /internal/shared/preload/0-sqlite.php e constants.php, per determinare perché ABSPATH non è disponibile prima dell'avvio di WordPress. Il lavoro è completato quando latest si avvia correttamente e il workflow della recipe procede senza eseguire il downgrade del runtime predefinito.

Scritto dal modello di indicizzazione a partire dal testo della issue.