fix(AIP-123): handle errant slash in pattern parsing (#1517)

When parsing the resource name `pattern`s to evaluate if a resource is a root/top-level resource, and pull out the parent resource if not, there was a bug where a leading slash on a root/top-level resource would make it think the pattern was multi-segmented (because it just checks for how many segments separators the `pattern` has). This would result in following through to logic that expected multiple items in a slice when there was only one, resulting in an out of bounds panic. We can handle this by trimming leading/trailing separators from the pattern before evaluating the pattern as a root/top-level resource 

Updates #1514

Author: noahdietz

rules/aip0123/aip0123.go

@@ -96,7 +96,7 @@ func isRootLevelResource(resource *apb.ResourceDescriptor) bool {
 // root-level resource by checking how many segments it has - root-level
 // resource patterns have only two segments, thus one delimeter.
 func isRootLevelResourcePattern(pattern string) bool {
-	return strings.Count(pattern, "/") <= 1
+	return strings.Count(strings.Trim(pattern, "/"), "/") <= 1
 }
 
 // getParentIDVariable is a helper that returns the parent resource ID segment

rules/aip0123/aip0123_test.go

@@ -43,6 +43,26 @@ func TestIsNestedName(t *testing.T) {
 			},
 			want: false,
 		},
+		{
+			name: "top level with extra leading slash",
+			resource: &apb.ResourceDescriptor{
+				Type:     "example.googleapis.com/Project",
+				Pattern:  []string{"/projects/{project}"},
+				Singular: "project",
+				Plural:   "projects",
+			},
+			want: false,
+		},
+		{
+			name: "top level with extra trailing slash",
+			resource: &apb.ResourceDescriptor{
+				Type:     "example.googleapis.com/Project",
+				Pattern:  []string{"projects/{project}/"},
+				Singular: "project",
+				Plural:   "projects",
+			},
+			want: false,
+		},
 		{
 			name: "non-nested child collection",
 			resource: &apb.ResourceDescriptor{