feat: MPPM virtual player & scenario tools

Companion to unity-mcp-plugin 2.31.0. Adds tool definitions for the new
plugin endpoints:
- unity_mppm_list_players / activate_player / deactivate_player
- unity_mppm_create_scenario
and clearer descriptions on the existing unity_mppm_* scenario tools.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>