feat(ui): add admin UI for RBAC/ABAC policy engine

[WilliamChen000]

🔗 Related Issue

Closes #2019(Phase3)

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📝 Summary

Implements Phase 3 of #2019 - Admin UI for the centralized RBAC/ABAC Policy Engine.

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🧪 Verification

Check	Command	Status
Lint suite	make lint	passed
Unit tests	make test	passed

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📓 Notes (optional)

What was accomplished

• mcpgateway/templates/policy_partial.html - New admin UI template with:
  • Engine health status cards (Native/OPA/Cedar/MAC)
  • Native RBAC rules table with add/delete support
  • Policy tester/simulator (subject + action + resource → allow/deny result)
  • Cache statistics display
• mcpgateway/main.py - Wire PolicyDecisionPoint singleton into app.state at startup
• mcpgateway/admin.py - 7 new API routes under /admin/policy/*:
  • GET /admin/policy/partial
  • GET /admin/policy/rules
  • POST /admin/policy/rules
  • DELETE /admin/policy/rules/{rule_id}
  • POST /admin/policy/test
  • GET /admin/policy/health
  • GET /admin/policy/cache/stats
• mcpgateway/templates/admin.html - Policy Engine tab and panel in sidebar
• mcpgateway/static/admin.js - Tab loader and event delegation for policy UI
• Verified engine health cards show Native engine as Healthy
• Verified rules can be added and deleted via the UI
• Verified policy tester returns correct ALLOW/DENY decisions
• Verified cache stats display correctly

[crivetimihai]

Thanks @WilliamChen000. Will review the UI implementation and integration with the policy engine.

[gcgoncalves]

Hi, @ WilliamChen000, thanks for your PR. We recently switched our UI to a new pattern, replacing the admin.js file with multiple smaller modules. Would you refactor to the new pattern?