fix(python): resolve Linux runtime without helper

python/sdk-runtime/src/deepseek_harness_runtime/__init__.py

@@ -5,8 +5,8 @@ Two runtime carriers coexist under ``runtime/``, both injected by the repo's
 
 - **exe (production)**: single-file Node executables named
   ``dsh-jsonrpc-agent-pkg-<platform>-<arch>`` (platform in {linux, macos}, arch in
-  {x64, arm64}) plus a sibling ``-spawn-helper`` used by ``node-pty``; the
-  target machine needs no Node installation.
+  {x64, arm64}); macOS also uses a sibling ``-spawn-helper``. The target machine
+  needs no Node installation.
 - **node (dev-only)**: the full deploy closure under ``runtime/node/``
   (``package.json`` + ``node_modules/``), executed as ``node
   runtime/node/node_modules/@deepseek-ai/dsh-jsonrpc-demo/lib/bin.js`` on a
@@ -71,11 +71,11 @@ def bundled_default_config_path() -> Path:
 def bundled_runtime_path() -> Path:
     """Absolute path of the bundled single-file runtime executable for the current platform.
 
-    Raises FileNotFoundError when the platform is unsupported or the executable
-    has not been placed into this package; the message names the acquisition
-    routes (acquisition strategy is deliberately separate from this lookup
-    interface, so an on-demand download can replace it without touching
-    callers).
+    Raises FileNotFoundError when the platform is unsupported, the executable
+    has not been placed into this package, or the required macOS spawn helper is
+    missing; the message names the acquisition routes (acquisition strategy is
+    deliberately separate from this lookup interface, so an on-demand download
+    can replace it without touching callers).
     """
     tag = _current_platform_tag()
     path = bundled_package_dir() / "runtime" / f"dsh-jsonrpc-agent-pkg-{tag}"
@@ -84,12 +84,13 @@ def bundled_runtime_path() -> Path:
             f"deepseek-harness-runtime-bin is missing the runtime executable at {path}. "
             + _EXE_ACQUISITION_HINT
         )
-    helper = Path(f"{path}{SPAWN_HELPER_SUFFIX}")
-    if not helper.is_file():
-        raise FileNotFoundError(
-            f"deepseek-harness-runtime-bin is missing the node-pty spawn helper at {helper}. "
-            + _EXE_ACQUISITION_HINT
-        )
+    if tag.startswith("macos-"):
+        helper = Path(f"{path}{SPAWN_HELPER_SUFFIX}")
+        if not helper.is_file():
+            raise FileNotFoundError(
+                f"deepseek-harness-runtime-bin is missing the node-pty spawn helper at {helper}. "
+                + _EXE_ACQUISITION_HINT
+            )
     return path