fix(policy-news): retry malformed per-source provider responses by stage

[luke-n-alpha]

관찰

Production executions �ipol-policy-news-daily-4nna0lc and �ipol-policy-news-daily-lzu32bp both isolated source ID �28d45bea07b4672856cc462 as PermanentProviderError. The batch remained successful and two other sources stayed durable; no source text or secret is included here.

목표

• Record a safe stage-specific error code without raw model output.
• Retry only the malformed stage so completed Solar/verification/translation calls are not repeated.
• Keep the per-source isolation and all-failed nonzero exit behavior.
• Add deterministic tests for empty, truncated, and non-JSON provider content.
• Verify with a manual production execution before closing.