Preserve PHPUnit bootstrap integration process failures

## Problem

When `tests/playground-phpunit-bootstrap-failure.integration.test.ts` receives a failed recipe-run process with empty stdout, its catch path calls `JSON.parse("")`. That replaces the original CLI process failure with `SyntaxError: Unexpected end of JSON input`, hiding stderr, exit status, timeout, and command context needed to diagnose the runtime failure.

This occurred in the full smoke aggregate while verifying #2464 and #2466. The test passes standalone, so preserving the original failure is required to diagnose any recurrence under aggregate load.

## Desired outcome

Parse recipe-run output only when non-empty JSON exists. Return structured failed recipe output when available; otherwise rethrow the original process error unchanged.

## Acceptance criteria

- Empty or absent stdout does not enter `JSON.parse`.
- Structured recipe failure JSON remains the primary assertion input.
- Unstructured process failures retain their original diagnostics.
- The standalone integration and full smoke aggregate pass.

## AI assistance

GPT-5.6 Sol via OpenCode identified the masking parser after the aggregate failure and drafted this diagnostic repair under Chris Huber's direction.

Inizia da tests/playground-phpunit-bootstrap-failure.integration.test.ts e analizza il percorso catch che gestisce i processi recipe-run non riusciti. Esegui il test di integrazione autonomo e l’aggregato smoke completo. Il lavoro è completato quando il JSON strutturato dell’errore rimane disponibile, mentre uno stdout vuoto o assente preserva l’errore di processo originale e le relative informazioni diagnostiche.

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