QONTO-WP-0003-T01: MCP adapter skeleton on shared capability core

Add a streamable-HTTP MCP adapter (mcp_server.py, official FastMCP SDK)
mounted at /mcp in the existing FastAPI app, with a combined lifespan so
the MCP session manager starts/stops with the service. Ships one smoke
tool (qonto_ping, no bank call) — real capability tools land in T02.

Prove REST and MCP hit the identical policy path: PolicyEngine.decide()
never branches on request.protocol, verified by a parametrized test.

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

tests/test_policy.py

@@ -1,5 +1,7 @@
 from pathlib import Path
 
+import pytest
+
 from qonto_assistant.contracts import ActorClaims, CapabilityRequest
 from qonto_assistant.policy import PolicyEngine
 
@@ -85,3 +87,34 @@ def test_policy_enforces_scope_when_enabled() -> None:
     decision = _policy(enforce_scope=True).decide(request)
     assert decision.allowed is False
     assert decision.reason == "authz_denied"
+
+
+@pytest.mark.parametrize("protocol", ["rest", "mcp"])
+def test_policy_decision_identical_across_protocols(protocol: str) -> None:
+    """QONTO-WP-0003-T01: REST and MCP must hit the identical policy path.
+
+    The decision for a known-deny case (a spend-shaped capability id) must
+    not depend on which transport originated the request.
+    """
+    claims = ActorClaims(
+        actor_id="agent-1",
+        tenant_id="binky",
+        lane="green",
+        scopes=frozenset({"finance.qonto.read"}),
+    )
+    request = CapabilityRequest(
+        capability_id="list_transactions",
+        tenant_id="binky",
+        actor_claims=claims,
+        resource_scope="test",
+        request_args={
+            "page": 1,
+            "page_size": 50,
+            "window_days": 31,
+            "requested_action": "transfer_funds",
+        },
+        protocol=protocol,
+    )
+    decision = _policy().decide(request)
+    assert decision.allowed is False
+    assert decision.reason == "spend"