feat: make failureThreshold nullable and skip defaulting

[t0mmylam]

Makes failureThreshold nullable in DeploymentPolicy strategies, allowing rollouts to continue indefinitely despite consecutive batch failures when the field is omitted.

Changes

• API Types: Removed failureThreshold from default value assignment in webhook
• Logic: Updated getFailureThreshold() to return *int and modified EvaluateBatchResult() to only stop rollouts when failureThreshold is explicitly set
• Docs: Clarified that DeploymentPolicy controls all node updates (not just interrupts) and documented nullable failureThreshold behavior
• Samples: Updated example YAMLs to demonstrate both explicit and omitted failureThreshold use cases
• Tests: Updated test expectations to verify failureThreshold remains nil after defaulting