perf: batch runs-service NOTIFY payloads

[Rohithmatham12]

Summary

• batch drained runs-service NOTIFY payloads into a single multi-call pg_notify statement per batch
• cap each batch at 1024 payloads to stay far below PostgreSQL's bind parameter limit
• retry failed NOTIFY batches instead of dropping all payloads from a transient connection error
• add unit coverage for query generation and batch draining/capping

Fixes #7756.

Testing

• go test ./runs/repository/impl -run 'Test(BuildNotifyBatchQuery|DrainNotifyBatch|NotifyActionUpdate_PayloadWithSpecialChars|NotifyRunUpdate_PayloadWithSpecialChars|RunNotifyLoop_NilConnNoPanic)$'