refactor: make listRoots() return unmodifiable list and add comprehensive tests

- Refactor McpAsyncServerExchange.listRoots() to return Collections.unmodifiableList
- Fix typo in variable name (allRootssResult -> allRootsResult)
- Improve code formatting and readability with ternary operator
- Add comprehensive test suite for McpAsyncServerExchange covering:
  - listRoots() pagination scenarios and edge cases
  - Logging notification with level filtering
  - Elicitation creation with various capabilities
  - Message creation with sampling capabilities
  - Error handling and validation scenarios

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