Support running test cases in offline environments

Description:

Several test cases in mcp-test and other src/test rely on external HTTP endpoints, which cause test failures in offline CI/CD environments.

For example, test case testToolCallSuccess of AbstractStatelessIntegrationTest.java throws an AssertionError if a GET request to raw.githubusercontent.com/modelcontextprotocol/java-sdk/refs/heads/main/README.md has null or empty response.

Goal:

• Support running maven test lifecycle for offline environments.
• Specifically, all test cases shall pass when -o option is appended to mvn test command, provided that all required docker images have already been imported (the default container pull policy will allow this to work offline).

Proposed changes:

Instead of sending GET request using an HTTP client, use a mock client that reads README.md from local file.

HttpClient mockClient = mock(HttpClient.class);
HttpResponse<String> mockResponse = mock(HttpResponse.class);

// Relative path of README.md depending on project directory
when(mockResponse.body()).thenReturn(Files.readString(Path.of("../README.md").toAbsolutePath()));
when(mockResponse.statusCode()).thenReturn(200);
when(mockClient.send(any(HttpRequest.class), any(HttpResponse.BodyHandler.class))).thenReturn(mockResponse);

Environment:

• MCP Java SDK: current main branch
• JDK: 21
• OS: Linux

Bắt đầu với testToolCallSuccess trong AbstractStatelessIntegrationTest.java và kiểm tra các test liên quan trong mcp-test và src/test để tìm các endpoint HTTP bên ngoài. Chạy vòng đời test của Maven với -o sau khi các image Docker cần thiết đã sẵn sàng. Hoàn tất khi bộ test chạy thành công trong môi trường offline mà không liên hệ với các endpoint bên ngoài.