fix(models): surface client-construction failures as factory misconfig

SDK client construction can fail on environment problems the config
never sees (e.g. httpx resolving a certifi CA path deleted by a venv
rebuild). Those escaped as bare exceptions and turned every workstream
open/create into an opaque 500; re-type them as ValueError in
ModelRegistry.get_client so routes answer 503 with the message and the
alias.

(cherry picked from commit 9c2e809b26)

tests/test_model_registry.py

@@ -167,6 +167,38 @@ class TestModelRegistry:
         with pytest.raises(ValueError, match="Unknown model alias"):
             reg.get_client("nonexistent")
 
+    def test_client_construction_failure_is_value_error(self) -> None:
+        # Environment failures inside SDK construction (e.g. httpx raising
+        # FileNotFoundError for a CA bundle deleted by a venv rebuild) must
+        # surface as ValueError so routes answer 503-with-message instead
+        # of an opaque 500.
+        reg = self._make_registry()
+        with (
+            patch(
+                "turnstone.core.model_registry.create_client",
+                side_effect=FileNotFoundError(2, "No such file or directory"),
+            ),
+            pytest.raises(ValueError, match="'default'.*FileNotFoundError") as excinfo,
+        ):
+            reg.get_client("default")
+        assert isinstance(excinfo.value.__cause__, FileNotFoundError)
+        # Nothing half-constructed may be cached — a later call with a
+        # repaired environment must construct for real.
+        assert "default" not in reg._clients
+
+    def test_client_construction_value_error_passes_through(self) -> None:
+        # create_client's own misconfig ValueErrors already carry
+        # remediation text and must not be double-wrapped.
+        reg = self._make_registry()
+        with (
+            patch(
+                "turnstone.core.model_registry.create_client",
+                side_effect=ValueError("anthropic-compatible requires base_url"),
+            ),
+            pytest.raises(ValueError, match="^anthropic-compatible requires base_url$"),
+        ):
+            reg.get_client("default")
+
     def test_shutdown(self) -> None:
         reg = self._make_registry()
         reg.get_client("default")

turnstone/core/model_registry.py

@@ -145,9 +145,25 @@ class ModelRegistry:
                 raise ValueError(f"Unknown model alias: {alias}")
             if alias not in self._clients:
                 cfg = self._models[alias]
-                self._clients[alias] = create_client(
-                    cfg.provider, base_url=cfg.base_url, api_key=cfg.api_key
-                )
+                try:
+                    self._clients[alias] = create_client(
+                        cfg.provider, base_url=cfg.base_url, api_key=cfg.api_key
+                    )
+                except ValueError:
+                    # create_client's own misconfig errors already carry
+                    # remediation text — pass through untouched.
+                    raise
+                except Exception as exc:
+                    # SDK construction can fail on environment problems the
+                    # config never sees — e.g. httpx resolving a CA-bundle
+                    # path that a venv rebuild deleted (FileNotFoundError).
+                    # Routes map ValueError to a 503 with the message;
+                    # anything else surfaces as an opaque 500, so re-type
+                    # here where the alias is known.
+                    raise ValueError(
+                        f"failed to construct {cfg.provider} client for model "
+                        f"alias {alias!r}: {type(exc).__name__}: {exc}"
+                    ) from exc
             return self._clients[alias]
 
     def get_provider(self, alias: str) -> LLMProvider: