chore: helm_release do not allow duplicate set values

[henkka]

The Helm provider currently (v.2.17.0 and v3.0.0-pre2) allows multiple set blocks with the same name (e.g., two set blocks with name = "defaultSSLPolicy") just like Helm. However, with the Terraform provider this creates unpredictable and non-intuitive behavior when configuring Helm releases.

Specifically:

1. When multiple blocks set the same name, the behavior is inconsistent and not aligned with user expectations
2. Users cannot reliably predict which value will be used when the same name appears multiple times
3. The order of blocks in the Terraform code doesn't determine which value takes precedence

This leads to confusing behavior where users might expect later blocks to override earlier ones (as Helm works), but instead, the result is unpredictable from the user's perspective.

For example:

resource "helm_release" "example" {
  # ...
  
  set = [
    {
      name  = "defaultSSLPolicy"
      value = "ELBSecurityPolicy-TLS13-1-2-2021-06"
    }

    {
      name  = "defaultSSLPolicy"
      value = "ELBSecurityPolicy-TLS13-1-2-Res-2021-06"
    }
 
  ]
}

In this case, which SSL policy will be used is not deterministic from the user's perspective, making it difficult for users to control their configuration.

Solution

This PR changes the behavior so it explicitly validate that each name is only used once, providing a clear error message if duplicates are found.

This makes the behavior more intuitive and explicit, helping users avoid subtle configuration errors.

I'd consider this breaking change, but figured I'd submit it anyway as you're about to release v3.0.0 of the provider, so this could fit there.

[hashicorp-cla-app]

All committers have signed the CLA.