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

Start with testToolCallSuccess in AbstractStatelessIntegrationTest.java and inspect the related tests under mcp-test and src/test for external HTTP endpoints. Run the Maven test lifecycle with -o after required Docker images are available. Done means the test suite passes in an offline environment without contacting external endpoints.