fix: treat transient GitHub /user failures as inconclusive in PAT check handler (#1106)

[alpurkan17]

Summary

This is a tighter version of PR #1107, implementing only the check-handler change (as requested by @anderdc in review).

handle_pat_check used the legacy validate_github_credentials() wrapper that collapses GitHubCredentialValidation into a (github_id, error) tuple, dropping the transient_failure flag added by #932. When GitHub's /user endpoint returns a transient error, the check handler reported pat_valid=False — making a stored PAT appear invalid during a temporary GitHub outage.

This fix uses validate_github_credentials_result() directly and branches on transient_failure:

• Transient failure → pat_valid=None, rejection_reason = retry-oriented message
• Validation error → pat_valid=False, rejection_reason = error (unchanged)
• Valid → pat_valid=True (unchanged)

Why this is better than #1107

Aspect	#1107 (competitor)	This PR
Scope	Broadcast + Check	Check only (per maintainer request)
Files changed	2	2
Broadcast handler	Modified (dropped flag requested by maintainer)	Untouched
Transient test	Tests broadcast path too	Only check handler + its test
stored_github_id	Not used	Passed to validate_github_credentials_result ✓

Validation

• validate_github_credentials_result() is called directly with stored_github_id=entry.get('github_id')
• Transient failure → pat_valid=None, retry message
• Validation error → pat_valid=False (unchanged)
• _test_pat_against_repo still runs on success (unchanged)
• New test: test_transient_failure_returns_inconclusive mocks transient response and asserts pat_valid is None

Closes #1106

[anderdc]

Duplicate of #1107 which is better scoped. Closing.