Add task-level encryption scope support for private session payloads

[cortensor-ryuma]

This PR adds backend support for task-level encryption scope while preserving existing session-level behavior as the default.

• Add backend support for task-level encryption scope while keeping session-level encryption as the default
• Track task-level encryption scope in PrivacySettingData with fresh-session-only enablement
• Update router payload encryption to derive and store task-scoped metadata correctly for off-chain v2 requests
• Keep miner decrypt/result handling aligned with encrypted payload scope metadata
• Preserve backward compatibility by falling back to session-scope behavior when older contracts do not expose the new task-scope getter