Add pytest scaffolding + first batch of unit tests for utils/helpers.py

[andrewtclim]

Summary

• Adds the first tests/ directory in the repo, with pytest configured as a dev dependency in pyproject.toml and discovery pointed at tests/.
• Writes six unit tests against src/deployml/utils/helpers.py, deliberately chosen to cover three distinct testing patterns rather than six variations of one:
  • Pure function — generate_bucket_name (2 tests: normal project ID; ID with underscores → hyphens for GCS validity)
  • Mocking the OS boundary — check_command (2 tests: binary found, binary missing — shutil.which patched via unittest.mock)
  • Parsing logic — estimate_terraform_time (2 tests: Cloud SQL detected → 20-min estimate; small non-SQL stack → resource-count estimate)
• All six tests pass locally in <2s.

Why this PR

The repo has no test scaffolding yet. Rather than add a large suite all at once, this PR establishes the infrastructure and demonstrates the three patterns future tests will lean on. Subsequent batches can extend tests/test_helpers.py (or split into more files) without touching the scaffolding.

Marking as draft so reviewers can weigh in on testing conventions (file layout, naming, fixture style) before more tests land.

Test plan

• python3 -m pytest -v runs and reports 6 passed
• python3 -m pytest -k bucket runs only the 2 bucket tests
• Reviewer confirms tests/ layout + pyproject.toml config match the conventions they'd like to see going forward

Notes / follow-ups

• No CI workflow added yet — intentional for this first PR. A GitHub Actions job that runs pytest on every PR is a small follow-up.
• No coverage tool (pytest-cov) added — also intentional, to keep the first batch focused.
• Next testing targets discussed: copy_modules_to_workspace (will need a tmp_path fixture), bucket_exists / generate_unique_bucket_name (will need to mock the google.cloud.storage client).