Apply Critical-Variable Availability Check at Intent Execution Time

Network Intent currently checks critical-variable availability before replication, preventing replication when required data is missing, but this same check does not apply at execution time. As a result, executing a replicated intent against an unreachable device or one that fails data collection causes the verification logic to treat missing data as a deviation, generating compliance warnings that do not reflect an actual configuration problem. This enhancement would extend the existing critical-variable availability check to execution, allowing execution to pass or return a distinct non-alerting status when required data cannot be retrieved, while reserving warnings for cases where data was collected and genuinely deviates from the intent. Consistent gating between replication and execution would reduce false positives caused by device unreachability, keep compliance alerts meaningful, and lower the manual effort required to distinguish unreachable devices from genuinely misconfigured ones.