get_cards(pathspec_string) silently resets the global client namespace to None #3395

metaflow.cards.get_cards() calls namespace(None) when it's given a task pathspec string (metaflow/plugins/cards/card_client.py, in get_cards). The call is global and never undone, so after get_cards("MyFlow/123/start/4") the caller's namespace is None. Every later client query in that notebook or script (Flow(...).latest_run, .runs(), Run(...)) then returns objects from all namespaces instead of the one the user picked.

Repro (local metadata/datastore), after running any flow with a @card step as another user:

from metaflow import Flow, namespace, get_namespace
from metaflow.cards import get_cards

namespace("user:someone-else")
get_cards("CardNsFlow/<run_id>/start/1")   # run owned by another user
print(get_namespace())                     # -> None (expected "user:someone-else")
print(Flow("CardNsFlow").latest_run)       # -> Run(...) instead of MetaflowNamespaceMismatch

Expected: get_cards finds the card but leaves the caller's namespace unchanged.

Proposed fix: replace the global namespace(None) with Task(task_str, _namespace_check=False), and pass _namespace_check=False on the two origin-task lookups used for resumed runs (get_cards and card_resolver._chase_origin). Those two lookups currently depend on the global None. This matches how Task.metadata already follows origin tasks. I have a small fix with a regression test and will link a PR.