fix(gateway): attach credential_pool to session /model overrides

Per-session /model overrides supplied api_key and provider but omitted
credential_pool, so billing rotation never ran on HTTP 402. Wire the pool
on fast override, rehydrate, and apply paths; backfill from provider for
legacy persisted overrides. Regression tests in tests/gateway/.

tests/gateway/test_session_model_override_credential_pool.py

@@ -0,0 +1,69 @@
+"""Session /model overrides must attach credential_pool for 402 rotation."""
+
+from __future__ import annotations
+
+from unittest.mock import MagicMock
+
+from gateway.run import GatewayRunner, _credential_pool_for_provider
+
+
+def test_fast_session_override_includes_credential_pool(monkeypatch):
+    runner = object.__new__(GatewayRunner)
+    runner._session_model_overrides = {
+        "sess-1": {
+            "model": "kimi-k2.7",
+            "provider": "custom:hyper",
+            "api_key": "sk-test",
+            "base_url": "https://hyper.charm.land/v1",
+            "api_mode": "chat_completions",
+        },
+    }
+    fake_pool = object()
+
+    monkeypatch.setattr(
+        "gateway.run._resolve_gateway_model",
+        lambda _uc=None: "default-model",
+    )
+    monkeypatch.setattr(
+        "gateway.run._credential_pool_for_provider",
+        lambda provider: fake_pool if provider == "custom:hyper" else None,
+    )
+
+    model, runtime = runner._resolve_session_agent_runtime(session_key="sess-1")
+
+    assert model == "kimi-k2.7"
+    assert runtime.get("credential_pool") is fake_pool
+
+
+def test_apply_session_override_backfills_credential_pool(monkeypatch):
+    runner = object.__new__(GatewayRunner)
+    fake_pool = MagicMock(name="pool")
+    runner._session_model_overrides = {
+        "sess-2": {
+            "model": "kimi-k2.7",
+            "provider": "custom:hyper",
+            "api_key": "sk-test",
+        },
+    }
+    monkeypatch.setattr(
+        "gateway.run._credential_pool_for_provider",
+        lambda provider: fake_pool,
+    )
+
+    model, runtime = runner._apply_session_model_override(
+        "sess-2",
+        "default-model",
+        {"api_key": "old", "provider": "x"},
+    )
+
+    assert model == "kimi-k2.7"
+    assert runtime["credential_pool"] is fake_pool
+
+
+def test_credential_pool_for_provider_delegates(monkeypatch):
+    sentinel = object()
+    monkeypatch.setattr(
+        "gateway.run._resolve_runtime_agent_kwargs_for_provider",
+        lambda p: {"credential_pool": sentinel, "provider": p},
+    )
+    assert _credential_pool_for_provider("custom:hyper") is sentinel