Fix fail-closed backend authorization

[gina941111-debug]

Summary

• remove Streamlit session coupling from backend permission checks
• deny direct backend tool calls when no verified authorization context exists
• bind the Gateway-validated role only for the duration of tool execution
• add regression coverage for missing, nested, and admin authorization contexts

Validation

• 36 focused authorization/Gateway/LINE tests passed
• 331 full project tests passed
• git diff --check passed

Scope

This PR only addresses the fail-open authorization issue. Existing untracked local documentation and system files are not included.