QONTO-WP-0004-T04: live flex-auth + tenant-engine authorization gate

Replaces the config-only QONTO_ASSISTANT_ENFORCE_SCOPE cached-claim
check with two live-checked facts, per docs/SecurityPractice.md #4:

1. flex-auth POST /v1/check on finance.qonto.read for the calling
   actor/tenant (FlexAuthCheckClient, modeled on tenant-engine's own
   client for the same API). Registration lives in the flex-auth repo
   (examples/qonto-assistant/) -- rules + embedded tests verified with
   flex-auth test-policy/load-registry/check, and a live flex-auth
   serve hit by this exact client over real HTTP (not a mock).
2. tenant-engine's live capability-role lookup
   (GET /tenants/{id}/roles/live), denying unless the tenant currently
   holds one of QONTO_TENANT_ENGINE_REQUIRED_ROLES (default VEN,CUS) --
   optional and additive to the flex-auth check.

Both clients fail closed by construction (unreachable/malformed/non-2xx
all deny, never grant), matching FlexAuthCheckClient's existing
fail-closed philosophy elsewhere in the fleet. LiveAuthorizationGate
combines both and is wired into CapabilityService._execute ahead of
the internal policy kernel; off by default (no QONTO_FLEX_AUTH_URL
set) so existing deployments are unaffected until configured.

Verified beyond mocked unit tests: ran a real `flex-auth serve` loaded
with the registered policy, and a real tenant-engine instance seeded
with a VEN grant for tenant:friendly:binky, and exercised this repo's
actual FlexAuthCheckClient/TenantEngineClient/LiveAuthorizationGate
against both live processes over real HTTP -- allow for the correct
tenant, live_authz_denied for a mismatched tenant.

28 new unit tests (flex_auth_client, tenant_engine_client,
live_authorization_gate + CapabilityService integration). Full suite
-> 80 passed; REST/MCP smokes and compileall still clean.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_tenant_engine_client.py

@@ -0,0 +1,70 @@
+import httpx
+
+from qonto_assistant.tenant_engine_client import TenantEngineClient
+
+
+def _client(handler) -> TenantEngineClient:
+    return TenantEngineClient(
+        base_url="https://tenant-engine.example.test",
+        timeout_seconds=1,
+        transport=httpx.MockTransport(handler),
+    )
+
+
+def test_active_roles_returns_roles_on_200() -> None:
+    def handler(request: httpx.Request) -> httpx.Response:
+        assert request.url.path == "/tenants/tenant:friendly:binky/roles/live"
+        return httpx.Response(200, json={"tenant_id": "tenant:friendly:binky", "roles": ["VEN", "CUS"]})
+
+    roles = _client(handler).active_roles("tenant:friendly:binky")
+
+    assert roles == frozenset({"VEN", "CUS"})
+
+
+def test_active_roles_empty_when_no_roles() -> None:
+    def handler(request: httpx.Request) -> httpx.Response:
+        return httpx.Response(200, json={"tenant_id": "tenant:friendly:binky", "roles": []})
+
+    assert _client(handler).active_roles("tenant:friendly:binky") == frozenset()
+
+
+def test_active_roles_fails_closed_on_404_tenant_not_found() -> None:
+    def handler(request: httpx.Request) -> httpx.Response:
+        return httpx.Response(404, json={"detail": "tenant_not_found"})
+
+    assert _client(handler).active_roles("tenant:friendly:nobody") == frozenset()
+
+
+def test_active_roles_fails_closed_on_503_store_unavailable() -> None:
+    def handler(request: httpx.Request) -> httpx.Response:
+        return httpx.Response(503, json={"detail": "tenant_roles_unavailable"})
+
+    assert _client(handler).active_roles("tenant:friendly:binky") == frozenset()
+
+
+def test_active_roles_fails_closed_on_malformed_body() -> None:
+    def handler(request: httpx.Request) -> httpx.Response:
+        return httpx.Response(200, content=b"not json")
+
+    assert _client(handler).active_roles("tenant:friendly:binky") == frozenset()
+
+
+def test_active_roles_fails_closed_on_non_list_roles_field() -> None:
+    def handler(request: httpx.Request) -> httpx.Response:
+        return httpx.Response(200, json={"tenant_id": "tenant:friendly:binky", "roles": "VEN"})
+
+    assert _client(handler).active_roles("tenant:friendly:binky") == frozenset()
+
+
+def test_active_roles_fails_closed_on_connection_error() -> None:
+    def handler(request: httpx.Request) -> httpx.Response:
+        raise httpx.ConnectError("connection refused", request=request)
+
+    assert _client(handler).active_roles("tenant:friendly:binky") == frozenset()
+
+
+def test_active_roles_fails_closed_on_timeout() -> None:
+    def handler(request: httpx.Request) -> httpx.Response:
+        raise httpx.TimeoutException("timed out", request=request)
+
+    assert _client(handler).active_roles("tenant:friendly:binky") == frozenset()