fix(simulation): retry temporary provider proxy failures

tests/test_gaussian_pipeline_gateway.py

@@ -14,6 +14,7 @@ from k1link.simulation.gaussian_pipeline_gateway import (
     GaussianPipelineGateway,
     GaussianPipelineGatewayError,
     GaussianPipelineIntegrityError,
+    GaussianPipelineUnavailableError,
     _discover_bundle_members,
 )
 
@@ -256,6 +257,30 @@ def test_gateway_surfaces_bounded_provider_rejection_detail(tmp_path: Path) -> N
         ))
 
 
+@pytest.mark.parametrize("status_code", [502, 503, 504])
+def test_gateway_classifies_temporary_provider_proxy_failures_as_unavailable(
+    tmp_path: Path,
+    status_code: int,
+) -> None:
+    with (
+        GaussianPipelineGateway(
+            "http://gaussian.test",
+            _token_file(tmp_path),
+            transport=httpx.MockTransport(
+                lambda _request: httpx.Response(
+                    status_code,
+                    json={"error": "provider_unavailable"},
+                )
+            ),
+        ) as gateway,
+        pytest.raises(
+            GaussianPipelineUnavailableError,
+            match=rf"HTTP {status_code}.*provider_unavailable",
+        ),
+    ):
+        gateway.capabilities()
+
+
 def test_gateway_rejects_incomplete_lcc_bundle(tmp_path: Path) -> None:
     bundle = tmp_path / "bundle"
     bundle.mkdir()