Improve validation

[iRevive]

Use Custom Validators

var _ validator.List = checkModeValidator{}

type checkModeValidator struct {
	mode string
}

func (v checkModeValidator) Description(_ context.Context) string {
	return fmt.Sprintf("mode must be set to %s", v.mode)
}

func (v checkModeValidator) MarkdownDescription(ctx context.Context) string {
	return v.Description(ctx)
}

func (v checkModeValidator) ValidateList(ctx context.Context, req validator.ListRequest, resp *validator.ListResponse) {
	if req.ConfigValue.IsNull() || req.ConfigValue.IsUnknown() {
		// If code block does not exist, config is valid.
		return
	}

	modePath := req.Path.ParentPath().AtName("mode")

	var m types.String

	diags := req.Config.GetAttribute(ctx, modePath, &m)
	resp.Diagnostics.Append(diags...)

	if resp.Diagnostics.HasError() {
		return
	}

	if m.IsNull() || m.IsUnknown() {
		// Only validate if mode value is known.
		return
	}

	if m.ValueString() != v.mode {
		resp.Diagnostics.AddAttributeError(
			modePath,
			"Mode value invalid",
			v.Description(ctx),
		)
	}
}

func CheckMode(m string) validator.List {
	return checkModeValidator{
		mode: m,
	}
}