Handle misconfigured tools (#1054)

## Issue
#983

## Change
When we configure a tool by using a `Class` instance, we get an
exception because only object instances are allowed.


## General checklist
<!-- Please double-check the following points and mark them like this:
[X] -->
- [ ] There are no breaking changes
- [ ] I have added unit and integration tests for my change
- [ ] I have manually run all the unit and integration tests in the
module I have added/changed, and they are all green
- [ ] I have manually run all the unit and integration tests in the
[core](https://github.com/langchain4j/langchain4j/tree/main/langchain4j-core)
and
[main](https://github.com/langchain4j/langchain4j/tree/main/langchain4j)
modules, and they are all green
<!-- Before adding documentation and example(s) (below), please wait
until the PR is reviewed and approved. -->
- [ ] I have added/updated the
[documentation](https://github.com/langchain4j/langchain4j/tree/main/docs/docs)
- [ ] I have added an example in the [examples
repo](https://github.com/langchain4j/langchain4j-examples) (only for
"big" features)


## Checklist for adding new model integration
<!-- Please double-check the following points and mark them like this:
[X] -->
- [ ] I have added my new module in the
[BOM](https://github.com/langchain4j/langchain4j/blob/main/langchain4j-bom/pom.xml)


## Checklist for adding new embedding store integration
<!-- Please double-check the following points and mark them like this:
[X] -->
- [ ] I have added a `{NameOfIntegration}EmbeddingStoreIT` that extends
from either `EmbeddingStoreIT` or `EmbeddingStoreWithFilteringIT`
- [ ] I have added my new module in the
[BOM](https://github.com/langchain4j/langchain4j/blob/main/langchain4j-bom/pom.xml)


## Checklist for changing existing embedding store integration
<!-- Please double-check the following points and mark them like this:
[X] -->
- [ ] I have manually verified that the
`{NameOfIntegration}EmbeddingStore` works correctly with the data
persisted using the latest released version of LangChain4j

---------

Co-authored-by: LangChain4j <langchain4j@gmail.com>

langchain4j/src/main/java/dev/langchain4j/service/AiServices.java

@@ -322,6 +322,10 @@ public AiServices<T> tools(List<Object> objectsWithTools) { // TODO Collection?
         context.toolExecutors = new HashMap<>();
 
         for (Object objectWithTool : objectsWithTools) {
+            if (objectWithTool instanceof Class) {
+                throw illegalConfiguration("Tool '%s' must be an object, not a class", objectWithTool);
+            }
+
             for (Method method : objectWithTool.getClass().getDeclaredMethods()) {
                 if (method.isAnnotationPresent(Tool.class)) {
                     ToolSpecification toolSpecification = toolSpecificationFrom(method);

langchain4j/src/test/java/dev/langchain4j/service/AiServicesBuilderTest.java

@@ -1,11 +1,17 @@
 package dev.langchain4j.service;
 
+import dev.langchain4j.agent.tool.Tool;
+import dev.langchain4j.data.message.AiMessage;
 import dev.langchain4j.exception.IllegalConfigurationException;
+import dev.langchain4j.model.chat.ChatLanguageModel;
+import dev.langchain4j.model.chat.mock.ChatModelMock;
+import dev.langchain4j.model.output.Response;
 import dev.langchain4j.rag.RetrievalAugmentor;
 import dev.langchain4j.rag.content.retriever.ContentRetriever;
 import dev.langchain4j.retriever.Retriever;
 import org.junit.jupiter.api.Test;
 import org.mockito.Mockito;
+import org.mockito.Spy;
 
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.mockito.Mockito.mock;
@@ -100,4 +106,26 @@ public void testRetrievalAugmentorAndContentRetriever() {
         });
     }
 
+    static class HelloWorld {
+
+        @Tool("Say hello")
+        void add(String name) {
+            System.out.printf("Hello %s!", name);
+        }
+    }
+
+    interface Assistant {
+
+        Response<AiMessage> chat(String userMessage);
+    }
+
+    @Test
+    public void should_raise_an_error_when_tools_are_classes() {
+        ChatLanguageModel chatLanguageModel = ChatModelMock.thatAlwaysResponds("Hello there!");
+
+        assertThrows(IllegalConfigurationException.class, () -> AiServices.builder(Assistant.class)
+                .chatLanguageModel(chatLanguageModel)
+                .tools(HelloWorld.class)
+                .build());
+    }
 }