Fix bisect_model pytest dependency

[kiwigitops]

Summary

• stop bisect_model output verification from importing coremltools.converters.mil.testing_utils
• keep the small random-input and PSNR helpers in models/utils.py, so correctness checking does not require pytest
• add a regression that exercises the verification path while blocking the test utility import

Fixes #2419

Tests

• python -m pytest coremltools/test/ml_program/test_utils.py::TestBisectModel::test_verify_output_correctness_does_not_import_testing_utils -q
• python -m compileall -q coremltools/models/utils.py coremltools/test/ml_program/test_utils.py
• git diff --check origin/main..HEAD

[kiwigitops]

Quick follow-up on this. The bisect_model verification path still imports cleanly without pytest-only utilities after this change. Happy to adjust the regression coverage if needed.