fix(pagerduty): add severity for labels

[srikanthccv]

📄 Summary

Fixes #9536

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Note

Adds severity and lastSeen labels to threshold alert outputs and context link presets, and standardizes threshold name constants to lowercase (critical/error/warning/info).

• Alerts/Rules:
  • Append severity label (lowercased threshold name) to alert samples in ruletypes.BasicRuleThreshold.shouldAlert.
  • Use ruletypes.LabelLastSeen when emitting no-data alerts in threshold_rule.go.
  • Add/standardize constants in ruletypes/constants.go: lowercase threshold names and new label keys (severity, lastSeen).
• Context Links:
  • Expand PredefinedAlertLabels to include severity and lastSeen.
• Tests:
  • Update threshold_test.go to use new constants and assert presence of severity label.

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