Add tests for critical coverage gaps

[tedsmitt]

Summary

This PR adds unit tests for functions that previously had 0% test coverage, improving overall test coverage from 37.4% to 45.3%.

New tests added:

• TestGetContainerPort in aws_test.go - Tests port retrieval from task definitions with single/multiple containers and error handling
• TestCreateOpts and TestInputLocalPort in new select_test.go - Tests the helper functions for building selection options
• TestExecuteForwardRequiresRealClient in forward_test.go - Documents that executeForward cannot be properly unit tested due to a type assertion to *ecs.Client on line 33
• TestPreRunEValidation and TestGetVersion in new cmd/main_test.go - Tests CLI argument validation logic

Review & Testing Checklist for Human

• Review forward_test.go: The test documents a design limitation - executeForward() does a type assertion e.client.(*ecs.Client) which prevents mocking. The test expects and catches this panic. Consider if this is acceptable or if the production code should be refactored to use the interface.
• Verify TestInputLocalPort: This test relies on the existing test-mode check (flag.Lookup("test.v")) which returns hardcoded "42069". It tests the test-mode path, not actual user input.
• Run go test ./... -v locally to confirm all tests pass in your environment

Notes

• No production code was modified, only test files added
• Tests follow existing patterns and mock structures already in the codebase
• The executeForward function would need refactoring to be properly testable (remove the type assertion to concrete *ecs.Client)

Link to Devin run: https://app.devin.ai/sessions/e2c116ced07146ffa9443dfddb14c76f
Requested by: Ed Smith (@tedsmitt)

[tedsmitt]

Testing Devin out for a friend, closing.