Validate required string tool arguments before API calls

[dejaguarkyng]

Several MCP tools cast required string arguments directly from request arguments and let invalid or blank values reach the API client. Add local validation for required string inputs such as job_id and artifact_id so users get clear MCP tool errors before a REST request is attempted.

Acceptance criteria:

• get_job, cancel_job, get_job_logs, stream_job_logs, list_job_artifacts, and get_artifact_download_url reject missing or blank IDs locally.
• Error messages name the tool and the invalid argument.
• Unit tests cover missing, non-string, and blank values.