fix Ray runtime env no working dir issue

[kunling-anyscale]

What does this PR do?

Respect Ray Job runtime_env in PPO runs by fixing get_ppo_ray_runtime_env():

• Do not inject working_dir=None (omit key entirely if absent).
• Pass through working_dir from RAY_JOB_CONFIG_JSON_ENV_VAR when provided.
• Avoid overriding env vars already present in os.environ.

This prevents VERL from clobbering a valid job working_dir and stabilizes env-var merge semantics when users submit via Ray Jobs.

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[gemini-code-assist]

Code Review

This pull request addresses a bug in get_ppo_ray_runtime_env that caused the working_dir in Ray's runtime environment to be handled incorrectly. The changes ensure that an existing working_dir from the job configuration is respected and passed through, and that the key is omitted if not present, which fixes the issue of it being clobbered. The JSON parsing is also made more robust. I've added one comment to further improve the exception handling by making it more specific.

[gemini-code-assist]

It's a good practice to catch specific exceptions rather than the generic Exception. This prevents masking other unexpected errors and makes the code more robust. In this case, json.loads will raise a json.JSONDecodeError if the string is not valid JSON.

Suggested change

	except Exception:
	except json.JSONDecodeError:

[wuxibin89]

@kunling-anyscale Please format code with: https://github.com/volcengine/verl/blob/main/CONTRIBUTING.md#code-linting-and-formatting