Get Observability Logs Exceptions

Recent Flink-side exceptions for the entity's current job.

Pulls from the Flink REST /jobs/{jid}/exceptions endpoint, not ClickHouse — exceptions are JM state, not a log row stream. Returns exceptions=[] (NOT 404) when the entity has no live Flink job — a draft agent / KB and a recently-cancelled one are both valid scopes with nothing to surface.