Add mtproxy phase contract and update checks

Introduce a formal mtproxy phase contract (Tools/mtproxy_phase_contract.py) describing phase metadata and utilities. Add a new guard script (Tools/check_mtproxy_phase_contract.py) to validate the contract against Java/native/analyzer code. Update multiple checking tools to consume the contract (check_mtproxy_all.py, check_mtproxy_connection_pattern_modes.py, check_mtproxy_endpoint_resilience_layers.py, check_mtproxy_resilience_contract.py, check_proxy_check_diagnostics.py, check_proxy_check_scheduler.py, check_proxy_connection_live_stages.py) and simplify/centralize phase expectations. Modify ConnectionsManager.java to compute stageTargetsCurrentProxy, mark endpoint failures earlier (before selected-account UI filtering) when rotation should be accelerated, and keep existing UI/usable logic intact.