🧪 test: implement unit testing for fleet HasCritical

[jackby03]

🎯 What: The testing gap for the fleet.HasCritical function has been addressed by introducing unit tests in internal/fleet/fleet_test.go. Previously, this pure function responsible for extracting critical severity occurrences from HostResult outputs lacked testing.

📊 Coverage: The test cases using table-driven logic now cover:

• Empty result sets
• Results missing the "critical" JSON key
• Results formatted as "critical":0 and "critical": 0 (which correctly return false)
• Results formatted as "critical": 1 or with extra whitespace (which correctly return true)
• Mixed results (results with no criticals and some with criticals)
• Checking that results with a non-nil Err field (non-OK results) are appropriately ignored, avoiding false positives.

✨ Result: The HasCritical logic is now fully covered by deterministic tests, providing a robust safety net against future refactoring or logic changes to error parsing in the fleet module.

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PR created automatically by Jules for task 13630435664046139039 started by @jackby03

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