feat(feature_flags): validate tenant ids, pin tenant flag behaviour (#375) [stack 4/11]

[antosubash]

Closes #375. Stack 4/11 of the tenancy-adoption series (base: #387).

What

The issue's premise turned out to be wrong: require_flag, is_flag_enabled and flag_enabled already resolve request.state.tenant_id. This PR pins that behaviour in tests and closes the remaining gap:

• Tenant id validation without coupling.
  • Core simple_module_core.tenancy.tenant_exists(app, id) reads app.state.tenant_exists, which tenants publishes. It returns None when no directory is installed.
  • feature_flags returns 404 for an unknown tenant on the list, set and browse override routes.
  • Clearing an override is deliberately not validated, so an override left behind for a deleted tenant can still be removed.
• Tests (16 tenancy + 2 core):
  • Tenant override precedence through require_flag for two tenants.
  • Fallback to the system value and then the default.
  • Boot hydration under strict mode, with no tenant bound.
  • No tenant role holds the flag permissions.
  • Unknown and known tenant ids are handled correctly.

Tests

make test-py: 3361 passed. make lint is clean.

https://claude.ai/code/session_01F8RiTBUJQnZmSq56qReZeV

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Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
🔒 Security Review	✅ Completed 2026-10-01T16:18:53.081283Z	355a6f9	Draft marked ready

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