fix: set parent attribute during IAM policy import (#184)

When importing a project-level IAM policy (e.g., `import { id = "projects/foo" }`),
`ImportStatePassthroughContext` only sets `d.SetId()` but not the `parent` attribute.
The Read function (`dataSourceIAMPolicyRead`) uses `d.Get("parent")` to determine
which resource to fetch. Since `parent` is empty, `ResolveWorkspaceParent` falls back
to the workspace, causing all project IAM policy imports to incorrectly read the
workspace IAM policy instead.

This fix replaces `ImportStatePassthroughContext` with a custom import function that
sets the `parent` attribute from the import ID before the Read function is called.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jun-kagawa

provider/resource_iam_policy.go

@@ -26,7 +26,12 @@ func resourceIAMPolicy() *schema.Resource {
 		UpdateContext: resourceIAMPolicyUpsert,
 		DeleteContext: resourceIAMPolicyDelete,
 		Importer: &schema.ResourceImporter{
-			StateContext: schema.ImportStatePassthroughContext,
+			StateContext: func(ctx context.Context, d *schema.ResourceData, m interface{}) ([]*schema.ResourceData, error) {
+				if err := d.Set("parent", d.Id()); err != nil {
+					return nil, err
+				}
+				return []*schema.ResourceData{d}, nil
+			},
 		},
 		Schema: map[string]*schema.Schema{
 			"parent": {