Migrate Fleet to Helm v4

[thardeck]

Upgrade Helm SDK from v3 to v4 and adapt Fleet to breaking API changes:

Changes:

• Replace deprecated fields and types:
  • DryRun (bool) -> DryRunStrategy enum (None=execute, Client=dry-run)
  • Force -> ForceReplace for resource replacement
  • Atomic -> RollbackOnFailure for rollback on upgrade failure
  • Removed ClientOnly (replaced by DryRunStrategy)
  • Removed Log method - migrate to controller-runtime's context-based logging
• Add required WaitStrategy configuration (StatusWatcherStrategy/HookOnlyStrategy)
• Fix ServerSideApply behavior:
  • Set to false when TakeOwnership=true to avoid managedFields conflicts
  • Use "auto" mode for upgrades to preserve initial apply method
• Add ContentCache field to downloader.Manager (required in Helm v4)
• Update drift correction tests to reflect Helm v4's improved client-side apply
• Refactor helmdeployer install() function to reduce cyclomatic complexity

Followup tasks:

• Do we want to stay with the Atomic option for Fleet or also support the Helm naming of RollbackOnFailure? At the moment Atomic is automatically mapped.
• Do we want to support OCI digest based chart installations, e.g. oci://registry.example.com/charts/app@sha256:abc123...?