Reinstate the @⁠Inject Technology Compatibility Kit (TCK)

In commit 05ebca8677, the `public` modifier was removed from the
SpringAtInjectTckTests class, which prevents it from being run as a
JUnit 3 test class.

To address that, this commit adds the missing `public` modifier as well
as a a code comment to help prevent this from happening again.

In addition, this commit updates spring-context.gradle to ensure that
the JUnit Vintage test engine is always applied. However, that Gradle
configuration is unfortunately ignored due to how our TestConventions
class has been implemented. Thus, that issue will have to be addressed
separately.

Closes gh-34800

spring-context/spring-context.gradle

@@ -59,3 +59,10 @@ dependencies {
 	testRuntimeOnly("org.javamoney:moneta")
 	testRuntimeOnly("org.junit.vintage:junit-vintage-engine") // for @Inject TCK
 }
+
+test {
+	description = "Runs JUnit Jupiter tests and the @Inject TCK via JUnit Vintage."
+	useJUnitPlatform {
+		includeEngines "junit-jupiter", "junit-vintage"
+	}
+}

spring-context/src/test/java/org/springframework/context/annotation/jsr330/SpringAtInjectTckTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -38,7 +38,8 @@ import org.springframework.context.support.GenericApplicationContext;
  * @author Juergen Hoeller
  * @since 3.0
  */
-class SpringAtInjectTckTests {
+// WARNING: This class MUST be public, since it is based on JUnit 3.
+public class SpringAtInjectTckTests {
 
 	@SuppressWarnings("unchecked")
 	public static Test suite() {