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.

Beginne in tests/playground-phpunit-bootstrap-failure.integration.test.ts und untersuche den catch-Pfad für die Verarbeitung fehlgeschlagener recipe-run-Prozesse. Führe den eigenständigen Integrationstest und das vollständige Smoke-Aggregat aus. Erledigt ist die Aufgabe, wenn strukturiertes Fehler-JSON weiterhin verfügbar bleibt, während leeres oder fehlendes stdout den ursprünglichen Prozessfehler und dessen Diagnoseinformationen bewahrt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.