refactor: improve MCP client code organization and robustness

- Add null parameter validation in client builders
- Enhance JavaDoc documentation for tools and resources
- Reorganize McpAsyncClient methods by functionality
- Fix sampling handler test setup

mcp/src/test/java/org/springframework/ai/mcp/client/AbstractMcpAsyncClientTests.java

@@ -22,6 +22,7 @@ import java.util.concurrent.atomic.AtomicBoolean;
 
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 import reactor.test.StepVerifier;
 
@@ -225,7 +226,8 @@ public abstract class AbstractMcpAsyncClientTests {
 			.hasMessageContaining("Root with uri 'nonexistent-uri' not found");
 	}
 
-	// @Test
+	@Test
+	@Disabled
 	void testReadResource() {
 		StepVerifier.create(mcpAsyncClient.listResources()).consumeNextWith(resources -> {
 			if (!resources.resources().isEmpty()) {

mcp/src/test/java/org/springframework/ai/mcp/client/McpAsyncClientResponseHandlerTests.java

@@ -365,7 +365,6 @@ class McpAsyncClientResponseHandlerTests {
 		// Create client with sampling capability but null handler
 		McpAsyncClient asyncMcpClient = McpClient.using(transport)
 			.capabilities(ClientCapabilities.builder().sampling().build())
-			.sampling(null) // Null handler
 			.async();
 
 		// Create a mock create message request