feat: support `--output-dir` on `helm template`

[abcqdsdchina]

Background

helm template --output-dir <dir> writes each rendered manifest to a separate file under <dir> instead of concatenating to stdout. With --use-release-name, files go under <dir>/<release-name>/. Common use: GitOps workflows that commit rendered manifests to a repo.

Helm SDK exposes this as (*action.Install).OutputDir (install.go:93) — template internally uses an Install action with ClientOnly=true.

Acceptance criteria

• TemplateCommand.withOutputDir(Path dir) writes each manifest to a separate file under dir
• TemplateCommand.useReleaseName() toggles the <dir>/<release-name>/ nesting (matches helm CLI's --use-release-name)
• Without withOutputDir, current behavior is preserved (concatenated YAML returned as string)
• Test asserts the expected files exist under the output dir and contain the expected manifests

Proposed Java API

helm.template()
    .withOutputDir(Paths.get("/tmp/rendered"))
    .useReleaseName()
    .call();
// → files like /tmp/rendered/release-name/templates/deployment.yaml

Implementation guide

Reference impl to copy patterns from: existing TemplateCommand + template.go (which delegates to install(...) internally).

Go (native/internal/helm/install.go + template.go)

• Add OutputDir string and UseReleaseName bool to InstallOptions struct (internal Go type). Only template will set these; install ignores them — that's fine.
• In install(...), set client.OutputDir = options.OutputDir and client.UseReleaseName = options.UseReleaseName.
• In template.go, plumb the new fields from TemplateOptions → the internal InstallOptions call.

CGO bridge (native/main.go)

• Add char* outputDir; and int useReleaseName; to struct TemplateOptions. Field order must match the JNA struct.
• Pass through to helm.TemplateOptions{OutputDir, UseReleaseName}.

JNA + Java

• Add public String outputDir; public int useReleaseName; to lib/api/.../TemplateOptions.java. Update @Structure.FieldOrder and constructor.
• Add withOutputDir(Path) and useReleaseName() builders to TemplateCommand.

Tests

• Add to HelmTemplateTest.
• Use @TempDir for the output dir.
• Scenarios:
  • withOutputDir: after .call(), assert files exist (e.g. dir/templates/<chart-name>/...)
  • withOutputDir + useReleaseName: assert files are nested under <release-name>/
  • No output dir: stdout return value still contains all manifests (existing behavior unchanged)

Contributor checklist

• Java 8 syntax only
• Apache 2.0 header
• @author Javadoc tag
• DCO sign-off
• make build-native && ./mvnw test -pl helm-java -Dtest=HelmTemplateTest green

References

• Helm CLI: https://helm.sh/docs/helm/helm_template/
• Helm SDK: https://github.com/helm/helm/blob/v3.21.0/pkg/action/install.go#L93 (OutputDir)
• helm CLI behavior: https://github.com/helm/helm/blob/v3.21.0/cmd/helm/template.go#L115-L120
• Project workflow: AGENTS.md → Adding a new Helm command