feat: add pytest-cov for coverage calculation

[swj9707]

Checked other resources

• This is a feature request/enhancement for Act Operator CLI, not a bug report.
• I added a clear, concise, and descriptive title for the proposal.
• I searched existing issues/PRs and did not find a duplicate.

💪 Ownership Declaration (Optional)

• 🙋 Hand-raise: I want to implement this feature myself
• 🤝 Collaboration Welcome: I want to work on this with others
• 💡 Idea Only: Proposing for someone else to implement

💡 What do you want?

Act operator doesn't have pytest-cov set up.
So, it can't measure test coverage, which can cause maintainability issues.

I found this Issue when I implement new feature, act-migrate.

💻 Example CLI or Code (Optional)

No response

📸 Screenshots or Output (Optional)

No response

[swj9707]

Before I started working on it, I wrote down why this is necessary.

Why Act should improve Test coverage

Act Operator is positioned as a production-ready CLI, but test coverage around act_operator/cli.py is still limited.

Currently, tests focus mainly on a narrow act new happy path,
leaving many user-facing flows untested, including:

• explicit act new command behavior
• most of the act cast flow
• interactive validation loops
• filesystem and error-handling branches

Since cli.py already handles command wiring, validation, and filesystem orchestration, adding new features (e.g., migrate) will further increase complexity.
Without stronger coverage, small changes may break CLI behavior without detection.

This issue aims to establish a regression safety net before expanding the CLI surface.

Current Baseline

uv run pytest --cov=act_operator --cov-report=term-missing

• act_operator/cli.py: 44%
• act_operator/utils.py: 49%
• total package coverage: 45%

Current tests mainly cover project initialization happy paths.

Scope

Priority 1

• explicit act new command behavior
• act cast happy path
• invalid project validation
• duplicate / non-empty cast directory handling

Priority 2

• interactive validation loops
• language selection & invalid input handling
• path resolution edge cases

Priority 3

• filesystem failure branches
• cast directory normalization
• cookiecutter / drawkit error handling

Expected Outcome

• stronger regression protection for CLI behavior
• safer refactoring of cli.py
• confidence before adding new commands (e.g., migrate)
• clearer separation between integration tests and unit/branch tests