Engineering team fixes

[d215steinberg]

1. Beefed up agent and task definitions to prevent backend_agent from writing front-end code and unit tests.
2. Explicitly specify UTF-8 encoding (not the default for Windows - Green check and red x were not showing up
3. Append stderr to test output and specify that output consists of just the test summary (previously, test_summary.md contained the test code.
4. Exclude Docker's .venv from the bind-mounted sandbox to prevent Windows-hostile Linux symlinks from seeping in.
5. Remove Sandbox directory on error for Windows to prevent starting up with corrupt sandbox
6. Specify no-sync on uv add gradio when resetting Sandbox to prevent Windows lockout.