fix: add pytest-json-report dependency and improve error handling

- Add pytest-json-report to requirements-test.txt for JSON test output
- Improve error handling in test runner service to catch JSONDecodeError
  and fallback to parsing stdout when JSON report is unavailable

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

requirements-test.txt

@@ -4,6 +4,7 @@ pytest>=8.3.4
 pytest-cov>=6.0.0
 pytest-asyncio>=0.24.0
 pytest-mock>=3.14.0
+pytest-json-report>=1.5.0
 httpx>=0.28.1
 faker>=33.1.0
 pytest-repeat>=0.9.4