[VRP] Harden testcase safety check in long-lived bots

[PauloVLB]

Problem

Long-lived bots shouldn't be touching untrusted testcases locally (like during minimization). The old check used the UNTRUSTED_UTASK env var, which isn't reliable and leads to missed detections.

Proposed Solution

Harden check_handling_testcase_safe in uworker_io.py to use environment.is_uworker() instead. If a long-lived bot (is_uworker() is False) tries to handle an untrusted testcase, it will immediately exit with a fatal error.

Testing

Updated minimize_task_test.py, test_utils.py, and utask_helpers.py to default testcases to trusted=True so they don't trigger the new safety check during unrelated tests.

Validation

Due to the difficulty of testing this in the dev environment and the lack of real untrusted fuzzers in production, this will be tested in the production environment.