fix(ladon): surface and recover structured-output retry exhaustion

[bokelley]

Problem

The Ladon review for adcontextprotocol/adcp#6902 reproducibly completes its
analysis but never returns schema-validated findings. The job correctly fails
closed, but identical reruns cannot produce an actionable review.

Observed on both the original and refreshed PR heads:

• https://github.com/adcontextprotocol/adcp/actions/runs/32941799252
• https://github.com/adcontextprotocol/adcp/actions/runs/33041294428

Across the attempts, Claude Code reports:

• error_max_structured_output_retries
• 17-24 turns
• permission_denials_count: 1
• no buffered inline comments
• no top-level or inline PR review

Other Ladon reviews in the same repository and time window complete
successfully, so this appears specific to this review path rather than a
repository-wide outage. Refreshing the branch from current main did not
change the outcome.

Expected behavior

The reviewer should either return schema-valid { summary, findings } output
(including an empty findings array for a clean review) or expose a specific,
actionable failure explaining why it cannot do so.

Please preserve the current fail-closed behavior: missing findings must not
be treated as approval.

Useful diagnostic improvement

When permission_denials_count is nonzero and structured output is exhausted,
surface the denied tool/category in the job annotation without exposing
review content or secrets. That would distinguish a reviewer permission
regression from a model formatting failure.