Fix list_all_resend_emails: pagination cap, error handling, and empty-result caching

[Copilot]

Three correctness issues in list_all_resend_emails() addressed based on code review feedback.

What does the PR do?

• Pagination cap raised + truncated flag added: max_pages increased from 10 → 100. Uses Python's for-else to set truncated=True only when the cap is actually hit. Flag is stored in Redis and surfaced in all response paths — callers can detect silently incomplete results.

• Page error handling: Introduced page_error_occurred flag. On any mid-pagination exception, the function now returns success: False and skips caching entirely instead of writing and serving partial data for 5 minutes.

• Empty-result caching: Removed if total_fetched > 0 guard — empty indexes are now cached with the same 300 s TTL, preventing repeated Resend API calls on accounts with no email history.

Type of change

• Breaking Change
• Bug Fix
• New Feature

Linked Issue

Make sure you have

• Pulled from the default branch
• Documented your changes
• Linked the Issue
• Appointed a reviewer (if any)

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