fix(plugin): share the runtime file-transfer root and 404 uninstalled icon requests

The SDK binds one transfer root inode (mode 0700) and picks it per profile: a shared worker uses /tmp/lbp-rpc while a host without the profile env fell back to data/temp/lbp, so plugin icons and assets died with 'Invalid file transfer capability'. The host now pins the shared root, and a plugin absent from the resolved Workspace answers 404 instead of 500.

tests/unit_tests/plugin/test_plugin_file_transfer_root.py

@@ -0,0 +1,51 @@
+"""The host must share the Runtime's plugin file-transfer root in shared deployments.
+
+A mismatch leaves plugin icons and assets failing with
+"Invalid file transfer capability" because the SDK binds one directory inode per
+process (mode 0700) and picks the default from the runtime profile.
+"""
+
+from __future__ import annotations
+
+import os
+from importlib import import_module
+from types import SimpleNamespace
+
+from langbot_plugin.runtime.io.handler import SHARED_WORKER_FILE_STORAGE_DIR
+from langbot_plugin.runtime.security import PLUGIN_FILE_STORAGE_DIR_ENV
+
+
+def connector_module():
+    import_module('langbot.pkg.core.app')
+    return import_module('langbot.pkg.plugin.connector')
+
+
+def align(profile: str) -> None:
+    connector = connector_module()
+    connector.PluginRuntimeConnector._align_plugin_file_transfer_root(
+        SimpleNamespace(runtime_profile=profile)
+    )
+
+
+def test_shared_profile_pins_the_runtime_transfer_root(monkeypatch):
+    monkeypatch.delenv(PLUGIN_FILE_STORAGE_DIR_ENV, raising=False)
+
+    align('shared')
+
+    assert os.environ[PLUGIN_FILE_STORAGE_DIR_ENV] == SHARED_WORKER_FILE_STORAGE_DIR
+
+
+def test_explicit_operator_value_is_never_overridden(monkeypatch):
+    monkeypatch.setenv(PLUGIN_FILE_STORAGE_DIR_ENV, '/var/lib/custom-transfer')
+
+    align('shared')
+
+    assert os.environ[PLUGIN_FILE_STORAGE_DIR_ENV] == '/var/lib/custom-transfer'
+
+
+def test_oss_profile_leaves_the_default_alone(monkeypatch):
+    monkeypatch.delenv(PLUGIN_FILE_STORAGE_DIR_ENV, raising=False)
+
+    align('oss_dev')
+
+    assert PLUGIN_FILE_STORAGE_DIR_ENV not in os.environ