fix(sdk): gracefully handle missing proto stubs in __init__.py

Wrap the sandbox import in try/except so subpackages like
openshell.prover can be imported without a full Rust build.
Removes the conftest.py namespace hack that was working around this.

Signed-off-by: Alexander Watson <zredlined@gmail.com>

python/conftest.py

@@ -1,20 +0,0 @@
-# SPDX-FileCopyrightText: Copyright (c) 2025-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
-# SPDX-License-Identifier: Apache-2.0
-
-"""Root conftest for Python tests.
-
-Pre-register openshell as a namespace package so subpackage tests (e.g.,
-the prover) can run without the full Rust build that generates proto stubs.
-"""
-
-from __future__ import annotations
-
-import sys
-import types
-from pathlib import Path
-
-if "openshell" not in sys.modules:
-    _pkg = types.ModuleType("openshell")
-    _pkg.__path__ = [str(Path(__file__).resolve().parent / "openshell")]
-    _pkg.__package__ = "openshell"
-    sys.modules["openshell"] = _pkg

python/openshell/__init__.py

@@ -5,18 +5,23 @@
 
 from __future__ import annotations
 
-from .sandbox import (
-    ClusterInferenceConfig,
-    ExecChunk,
-    ExecResult,
-    InferenceRouteClient,
-    Sandbox,
-    SandboxClient,
-    SandboxError,
-    SandboxRef,
-    SandboxSession,
-    TlsConfig,
-)
+try:
+    from .sandbox import (
+        ClusterInferenceConfig,
+        ExecChunk,
+        ExecResult,
+        InferenceRouteClient,
+        Sandbox,
+        SandboxClient,
+        SandboxError,
+        SandboxRef,
+        SandboxSession,
+        TlsConfig,
+    )
+except ImportError:
+    # Proto stubs not yet generated (requires Rust build).
+    # Subpackages like openshell.prover can still be imported.
+    pass
 
 try:
     from importlib.metadata import version