feat(mlflow): add air-gap preflight checks for registry and image validation

Parent epic: #166
Integration branch: feat/mlflow-enterprise-patterns

Add preflight checks that validate air-gap readiness: registry reachability, image availability, and local mirror configuration.

Scope

• Add to release/kots-preflight.yaml:
  • Registry reachability check (can the cluster pull from the configured registry?)
  • Image pull validation for critical images (mlflow, cloudnative-pg, minio-operator)
  • Storage class validation for PVC-backed services
• Use appropriate collectors: registryImages or imagePull for registry validation
• Add analyzers with clear pass/warn/fail messages for air-gap scenarios
• Document the preflight checks in the main README.md (what they check and why)

Files touched

• applications/mlflow/release/kots-preflight.yaml (add air-gap collectors + analyzers)
• applications/mlflow/README.md (document new preflights)

PR target

Branch PRs to feat/mlflow-enterprise-patterns (not main).

Start by reading applications/mlflow/release/kots-preflight.yaml and identify its existing collectors and analyzers. Add the air-gap checks for registry reachability, critical image pulls, and storage classes, with clear pass, warn, and fail messages, then document what they check and why in applications/mlflow/README.md. Done means all listed checks and documentation are present for the target integration branch.