Harden operator heartbeat reconciliation

## Summary
Track the hardening in PR #1105 to keep operator heartbeats from stalling when Kusto management calls hang.
- add bounded timeouts around the partition heartbeat Kusto calls
- add stage-specific error context for heartbeat failures
- allow multiple ADXCluster reconciles to run in parallel so one stalled reconcile does not block the controller
- add focused regression coverage for the new heartbeat error wrapping

## Motivation
The current heartbeat flow couples liveness to schema discovery. This issue tracks the hardening in PR #1105, which makes the synchronous flow safer without changing the heartbeat payload contract.

## Related
- PR: https://github.com/Azure/adx-mon/pull/1105

Start with PR #1105 and trace the operator heartbeat flow, especially the partition heartbeat Kusto calls and ADXCluster reconciliation. Verify the bounded calls, stage-specific heartbeat error context, parallel reconcile behavior, and focused regression coverage for error wrapping; done means stalled management calls no longer block liveness without changing the heartbeat payload contract.

Written by the indexing model from the issue text.