Union template declaration were being finished in projector (#3052)

fix [#2946](#2946)

Problem was that in the projector were were calling to this helper to
see if that was a type that we should finish but it was missing the
union case causing union template declaration from being finished in the
projector and then being shown as finished type in the semantic
navigator and crashing openapi3 emitter.

.chronus/changes/fix-union-template-2024-2-26-16-22-2.md

@@ -0,0 +1,8 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Fix: Union template declaration were incorrectly being finished in projection

packages/compiler/src/core/type-utils.ts

@@ -50,6 +50,7 @@ export function getParentTemplateNode(node: Node): (Node & TemplateDeclarationNo
     case SyntaxKind.ModelStatement:
     case SyntaxKind.ScalarStatement:
     case SyntaxKind.OperationStatement:
+    case SyntaxKind.UnionStatement:
     case SyntaxKind.InterfaceStatement:
       return node.templateParameters.length > 0 ? node : undefined;
     case SyntaxKind.OperationSignatureDeclaration: