chore: migrate to Helm 4

[lexfrei]

Context

Helm 4.0.5 is available. Migration is deceptively simple — code compiles, existing tests pass, but behavior changes silently.

This is a trap. The API signatures mostly remain, but internal behavior differs. Without comprehensive tests, breakages will only surface in production.

Current State

Helm library usage in:

• pkg/engine/engine.go
• pkg/engine/helm/engine.go
• pkg/engine/helm/files.go

Current test coverage is insufficient to detect behavioral regressions.

Migration Strategy

Phase 1: Lock Current Behavior (BEFORE migration)

• Add comprehensive tests for template rendering
• Test edge cases: nested templates, includes, tpl function
• Test all custom template functions we use
• Test chart loading and dependency resolution
• Test values merging behavior
• Snapshot test outputs for known inputs

Phase 2: Migration

• Update helm.sh/helm/v3 → helm.sh/helm/v4
• Fix import paths (pkg/chart → pkg/chart/v2, etc.)
• Fix any API changes

Phase 3: Verify

• All Phase 1 tests must pass unchanged
• Any test failures indicate behavioral regression — investigate

Resources

• Helm 4 Changelog
• Helm 4 Released