Add unit tests for carrier-selection edge cases

[mkarson1997]

Goal

Turn the carrier-selection logic into an explicitly tested engineering artifact.

Completed

• Added dedicated CargoAPI.Tests xUnit project.
• Added Moq-based isolation for repository dependencies.
• Non-positive desi rejects without persistence.
• Exact lower boundary is covered.
• Exact upper boundary is covered.
• Multiple matching configurations select the cheapest carrier.
• Documented above-range extra-desi example is covered (13, max 10, base 32, plus 4 → 44).
• Missing carrier configuration fails without persistence.
• Unit tests run in GitHub Actions and the latest CI is green.
• README documents the automated test command.

Remaining edge cases

• Inactive carrier configurations at the repository/integration boundary.
• Equal-price/tie behavior if product policy requires deterministic selection.
• Below-all-ranges behavior after the domain rule is explicitly decided.
• Gap-between-ranges behavior after the domain rule is explicitly decided.

The two undefined pricing cases are tracked separately in issue #3 so tests do not accidentally freeze an unintended business rule.