Show retryable collection errors for malformed operator responses

[zackbart]

A successful /ui/data response containing null throws during validation; malformed artifact and access-token collection responses are treated as empty collections.

Required behavior and acceptance criteria:

• Malformed successful collection payloads enter the existing recoverable error state without hiding valid cached rows or crossing identity gates. Valid empty arrays remain successful.
• Add a regression reproducing the failure and verify the applicable Node and Workers suites.

Found during the post-merge reliability sweep following #643.