[codex] Use assert in Docker build test

[adamtheturtle]

Summary

Replace the explicit raise AssertionError(full_log) from exc in the Docker build test with an assert that still reports the full build log.
This keeps the unsupported Windows container skip behavior while making the assertion style simpler.

Checks

Pre-commit and pre-push hooks passed, including Ruff, mypy, pyright, ty, and pyrefly.
Also ran the Docker test under coverage successfully; the scoped coverage report for this single file remains 93% because the BuildError handler is not exercised on macOS after removing the pragma.

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Note

Low Risk
Low risk: only adjusts test failure/skip logic in the Docker build test, without changing runtime code paths.

Overview
Simplifies the Docker image build failure handling in tests/mock_vws/test_docker.py by replacing a manual raise AssertionError(...) with an assert that includes the full build log.

The test still skips when the BuildError matches known Windows-container manifest errors, but now fails with a standard assertion (and clearer log output) for unexpected build failures.

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