refactor: standardize test parameterization and fix package naming (#556)

- Replace @ValueSource with @MethodSource for parameterized tests
- Add clientsForTesting() methods to provide test arguments consistently
- Remove hardcoded Spring-related labels from framework-agnostic mcp-test module
- Move mcp-test's utility classes from io.modelcontextprotocol.utils to io.modelcontextprotocol.util

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>