Add boot42-preview module: Spring Boot 4.2.0-M2 vs 4.1.1 forwarded headers, AMQP starters, buildpack cache, RestTemplate removal warnings

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Gth99spB1CzQmBFofXmU4Z

boot42-preview/src/test/java/com/ankurm/boot42/BootRun.java

@@ -0,0 +1,32 @@
+package com.ankurm.boot42;
+
+import java.util.Map;
+import org.springframework.boot.WebApplicationType;
+import org.springframework.boot.builder.SpringApplicationBuilder;
+import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.core.env.Environment;
+import org.springframework.web.client.RestClient;
+
+/** Starts the real application with the given properties on a random port. */
+final class BootRun {
+
+	private BootRun() {
+	}
+
+	static ConfigurableApplicationContext start(String... props) {
+		return new SpringApplicationBuilder(Boot42PreviewApplication.class)
+				.web(WebApplicationType.SERVLET)
+				.properties(props)
+				.properties("server.port=0", "spring.main.banner-mode=off", "logging.level.root=WARN")
+				.run();
+	}
+
+	static RestClient client(ConfigurableApplicationContext ctx) {
+		Environment env = ctx.getEnvironment();
+		return RestClient.create("http://localhost:" + env.getProperty("local.server.port"));
+	}
+
+	static Map<?, ?> whoAmI(ConfigurableApplicationContext ctx, Map<String, String> headers) {
+		return client(ctx).get().uri("/whoami").headers(h -> headers.forEach(h::add)).retrieve().body(Map.class);
+	}
+}