Reactive compaction telemetry gains error-kind and cache-cold fields

What

• Reactive-compaction failure results now include a structured errorKind field (thrown, no_assistant_message, empty_summary, or the API's own error tag) alongside the existing free-text detail.
• The reactive-compaction telemetry event now also reports cacheCold (whether the prompt cache was cold) and a status field, threaded through as a new cacheCold option.

Why

This makes it easier to tell why an automatic (reactive) compaction attempt failed and whether a cold prompt cache played a role, improving diagnosis of compaction issues.