refactor(drm): resolve content keys in one licence path behind ServerKeySource

Drop null keys from every licence, server and export source, not only Widevine. dl now finds remote sessions and imports through ServerKeySource instead of private fields.

tests/remote/key_source.py

@@ -0,0 +1,23 @@
+from types import SimpleNamespace
+from typing import Any, Iterable, Optional
+from uuid import UUID
+
+from unshackle.core.remote_service import RemoteService, ServerVault
+
+
+def remote_keys(
+    server_cdm: Optional[bool] = True,
+    server_cdm_type: str = "widevine",
+    client_licensed: Iterable[str] = (),
+    local_cdm: Any = None,
+    server_vault_keys: Optional[dict[UUID, str]] = None,
+) -> RemoteService:
+    """A RemoteService with only the state dl reads through ServerKeySource, and no server."""
+    svc = RemoteService.__new__(RemoteService)
+    svc._server_cdm = server_cdm
+    svc.server_cdm_type = server_cdm_type
+    svc.client_licensed = set(client_licensed)
+    svc.server_vault_keys = dict(server_vault_keys or {})
+    svc.server_vault = ServerVault(svc)
+    svc.ctx = SimpleNamespace(obj=SimpleNamespace(cdm=local_cdm))
+    return svc