install.sh installs helm-diff v3.13.0 with incompatible Helm 4

[danehans]

Component

Setup/Standup

Describe the bug

At the tip of main (9b11b40929f1c4ffefedee7d52e6b8329a224b70), install.sh pins Helm v3.19.0 and helm-diff v3.13.0.

The Helm version check treats the Helm pin as a minimum, so an existing Helm v4.2.0 is accepted. The helm-diff block, however, enforces an exact version and reinstalls v3.13.0, even if a Helm 4-compatible newer plugin is already installed.

A subsequent helmfile apply against an existing release fails during helm diff upgrade:

Error: Failed to render chart: exit status 1: Flag --validate has been deprecated, use --dry-run=server instead
Error: if any flags in the group [validate dry-run] are set none of the others can be; [dry-run validate] were all set
Error: plugin "diff" exited with error

A first installation can appear successful because llm-d-benchmark passes --skip-diff-on-install. A rerun against the existing Helm release exercises helm-diff and fails.

Steps to reproduce

1. Install or upgrade to Helm v4.2.0.
2. Install a Helm 4-compatible helm-diff release, such as v3.15.7.
3. Run ./install.sh --uv.
4. Observe that helm plugin list now reports helm-diff 3.13.0.
5. Run a modelservice standup against an existing Helm release.
6. Observe the mutually exclusive validate and dry-run flag error above.

Expected: install.sh should ensure a compatible Helm/helm-diff combination. Possible approaches include selecting helm-diff based on the detected Helm major version, treating the helm-diff pin as a compatible minimum rather than an exact downgrade, or enforcing Helm 3 if that is the intended toolchain.

Additional context or screenshots

This is distinct from #909, which addressed Helm 4 plugin signature verification during installation. The plugin installs successfully here but fails when Helmfile invokes it.