fix: add ConnectClient import and instantiation to generated endpoints

Fixed TypeScript generation error "Cannot find name 'client'" by adding:
- Import of ConnectClient from @vaadin/hilla-frontend
- Instantiation of client constant in generated endpoint files

The generated endpoint methods use client.call() but the client was
never imported or created, causing TypeScript compilation errors.

Changes:
- Updated ClientPlugin.java to import ConnectClient alongside EndpointRequestInit
- Added `const client = new ConnectClient();` to generated files
- All 10 TypeScriptGeneratorTest tests pass

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: Artur-

packages/java/typescript-generator/src/main/java/com/vaadin/hilla/typescript/codegen/plugins/ClientPlugin.java

@@ -51,7 +51,7 @@ private String generateEndpointClient(ClassInfoModel endpoint) {
         TypeScriptWriter writer = new TypeScriptWriter();
 
         // Add imports
-        writer.addNamedImport(List.of("EndpointRequestInit"),
+        writer.addNamedImport(List.of("ConnectClient", "EndpointRequestInit"),
                 "@vaadin/hilla-frontend");
 
         // Generate client methods for each public method in the endpoint
@@ -66,6 +66,8 @@ private String generateEndpointClient(ClassInfoModel endpoint) {
                 " * " + endpoint.getSimpleName() + " endpoint client.");
         writer.appendLine(" */");
         writer.appendBlankLine();
+        writer.appendLine("const client = new ConnectClient();");
+        writer.appendBlankLine();
         writer.append(methods);
 
         return writer.build();