test: add regression coverage for server startup imports

[aneek22112007-tech]

Problem

A recent issue caused the MCP server to fail at startup due to a broken tool import (cannot import name 'fit_predict_tool' from sktime_mcp.tools.data_tools). This happens when tools are refactored, renamed, or moved, but the main server.py file is not updated to reflect those changes.

Solution

This PR does two things:

1. Includes the fix for the broken fit_predict_tool import in server.py.
2. Adds tests/test_server_startup.py to act as a fast, isolated regression guard.
   • test_server_import_success: verifies import sktime_mcp.server succeeds without any ImportError.
   • test_server_tools_registration: validates that the underlying MCP server object properly initializes and has critical expected tools registered (e.g., fit_predict, load_data_source).

Testing

• Verified locally. All tests pass, including the new smoke tests.
• These tests are deterministic and run in milliseconds.