feat(12-02): normalize all four providers into CloudResourceAdapter contract

- GoogleDriveBackend: list_folder with full pagination, native doc size=None, get_capabilities
- OneDriveBackend: list_folder with @odata.nextLink pagination, get_capabilities
- WebDAVBackend: list_folder via PROPFIND with SSRF re-validation, get_capabilities
- NextcloudBackend: inherits CloudResourceAdapter from WebDAVBackend
- build_cloud_resource_adapter() added to factory
- 22 new contract/behavior/pagination/no-byte-download tests (51 total pass)

backend/storage/cloud_backend_factory.py

@@ -1,4 +1,7 @@
 """Factory for user-scoped cloud storage backends."""
+from __future__ import annotations
+
+from storage.cloud_base import CloudResourceAdapter
 
 
 def build_cloud_backend(provider: str, credentials: dict):
@@ -31,3 +34,22 @@ def build_cloud_backend(provider: str, credentials: dict):
         )
 
     raise ValueError(f"Unknown provider: {provider}")
+
+
+def build_cloud_resource_adapter(provider: str, credentials: dict) -> CloudResourceAdapter:
+    """Build a CloudResourceAdapter for the given provider and credentials.
+
+    Returns a provider instance that implements the CloudResourceAdapter read-only
+    interface (list_folder, get_capabilities, merge_item_capabilities). All four
+    supported providers implement CloudResourceAdapter as a mixin alongside
+    their StorageBackend interface.
+
+    Raises:
+        ValueError: If provider is not one of the four supported cloud providers.
+    """
+    backend = build_cloud_backend(provider, credentials)
+    if not isinstance(backend, CloudResourceAdapter):
+        raise ValueError(
+            f"Provider {provider!r} backend does not implement CloudResourceAdapter"
+        )
+    return backend