Add comprehensive unit tests for deploy command

[jasondaming]

Summary

Adds comprehensive unit tests for the robotpy deploy command, covering core functionality with 22 tests.

Changes

• tests/test_deploy.py: 22 unit tests covering deploy command functionality
• tests/TEST_COVERAGE.md: Documentation of test coverage and future expansion areas

Test Coverage

What's Tested (22 tests, all passing ✅)

• SSH error handling and context management (2 tests)
• File operations and filtering (6 tests)
  • Ignores: hidden files, .pyc, __pycache__, venv, .whl files
• Large file detection and user confirmation (3 tests)
• Build metadata generation with git integration (2 tests)
• Package caching and management (3 tests)
• Deploy workflow controls (3 tests)
  • Home directory blocking
  • Test execution control
  • Skip tests flag
• Command-line argument parsing (1 test)
• Integration test for complete deploy workflow (1 test)

Testing Strategy

• Uses extensive mocking to avoid requiring actual robot hardware
• Tests are isolated, fast, and include proper cleanup
• Tests both success and failure scenarios
• Follows Arrange-Act-Assert pattern

Running the Tests

# Run all deploy tests
python3 -m pytest tests/test_deploy.py -v

# Run with coverage
python3 -m pytest tests/test_deploy.py --cov=robotpy_installer.cli_deploy

Future Expansion

The TEST_COVERAGE.md document outlines areas for future test expansion including:

• SSH/Robot communication testing
• Requirements management flows
• Deploy execution edge cases
• Network failure scenarios

This PR establishes a foundation for testing the deploy command and provides patterns that can be used for testing other commands in robotpy-installer.

[virtuald]

I've generally found that extensive mocks in tests just make things annoying to modify in the future, so I'm inclined to not go forward with this. I was hoping the mocks would be less invasive, or maybe it would stand up a fake SSH server using paramiko, or just mock the SSH client.

Also, it definitely needs to be pytest-style test cases, not unittest.