Handle missing gRPC server event type in test initializer

[xfocus3]

Closes gh-50825

spring-boot-starter-test-classic can include spring-boot-grpc-test without the Spring gRPC server classes. The gRPC test initializer currently adds a listener whose generic event type references GrpcServerStartedEvent; refreshing any context then asks Spring to resolve the listener event type and fails when that class is not on the classpath.

This changes the listener to accept application events and only handle gRPC server-started events by class name. It keeps the existing port-property behavior when Spring gRPC is present and avoids resolving the event type when it is absent.

Verification:

• RED: GrpcPortInfoApplicationContextInitializerTests.whenGrpcServerStartedEventIsNotPresentInitializerDoesNotFailOnRefresh failed before the fix with TypeNotPresentException: Type org.springframework.grpc.server.lifecycle.GrpcServerStartedEvent not present
• ./gradlew --no-daemon :module:spring-boot-grpc-test:test --tests org.springframework.boot.grpc.test.autoconfigure.GrpcPortInfoApplicationContextInitializerTests
• ./gradlew --no-daemon :module:spring-boot-grpc-test:test :module:spring-boot-grpc-test:checkstyleMain :module:spring-boot-grpc-test:checkstyleTest
• git diff --check HEAD