fix: handle transient GitHub failures in validator PAT handlers (#1106)

[alpurkan17]

Description

Fix #1106 — The validator PAT handlers (handle_pat_broadcast and handle_pat_check) use the legacy validate_github_credentials() wrapper that drops the transient_failure flag. Transient GitHub /user failures are treated as invalid PATs instead of inconclusive.

Changes

• Replace validate_github_credentials() with validate_github_credentials_result() in both handlers
• handle_pat_broadcast: transient failure → reject with retry-oriented message
• handle_pat_check: transient failure → pat_valid=None (inconclusive) instead of pat_valid=False

Testing

• Transient GitHub /user failure → broadcast rejects with retry message
• Transient GitHub /user failure → check returns pat_valid=None
• Normal validation error → unchanged behavior
• Valid PAT → unchanged behavior

[anderdc]

Closing as duplicate of #1107