Add langchain4j-interop: Spring AI 2.0 and LangChain4j 1.22 side by side

Same order assistant in both frameworks against a local fake OpenAI
server: starter bean collision, wire differences, tool failure and loop
bounds, observability meters, MCP-first tools, and a compile check of
the 2025 article's snippets and Maven coordinates.

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01G8ikz8xdWuTP5yun8DZ1hk

langchain4j-interop/src/broken/OldChatService.java

@@ -0,0 +1,32 @@
+import java.util.List;
+
+import org.springframework.ai.chat.client.ChatClient;
+import reactor.core.publisher.Flux;
+
+// The original ChatService, unchanged: this one is expected to still compile.
+class OldChatService {
+    private final ChatClient chatClient;
+
+    OldChatService(ChatClient.Builder builder) {
+        this.chatClient = builder
+            .defaultSystem("You are a senior Java architect. Be precise and concise.")
+            .build();
+    }
+
+    public String ask(String userMessage) {
+        return chatClient.prompt().user(userMessage).call().content();
+    }
+
+    public Flux<String> stream(String userMessage) {
+        return chatClient.prompt().user(userMessage).stream().content();
+    }
+
+    public ArchitectureReview reviewCode(String codeSnippet) {
+        return chatClient.prompt()
+            .user(u -> u.text("Review this Java code for production readiness: {code}").param("code", codeSnippet))
+            .call()
+            .entity(ArchitectureReview.class);
+    }
+
+    public record ArchitectureReview(String verdict, List<String> issues, List<String> suggestions, int score) { }
+}