Increase behavioral test coverage to 70-80%

[timimsms]

Context

PR #16 brought test coverage from 16% to 51% by cherry-picking testing infrastructure from the coverage audit. The next step is reaching 70-80% with behavioral tests — tests that verify correct behavior, not just code execution.

Current coverage by package

Package	Coverage	Target
api	24.4%	60%+
auth	0%	70%+
auth/mock	79.7%	—
cache	35.1%	70%+
cmd	18.8%	40%+
config	69.5%	80%+
errors	89.7%	—
output	79.6%	—
version	100%	—

Approach

The testing infrastructure from PR #16 provides the foundation:

• Mock infrastructure (internal/auth/mock/, internal/mocks/)
• Interface definitions (internal/interfaces/)
• Test utilities (internal/testutil/)

Priority areas

1. auth package — currently 0%, needs mock keyring tests for save/get/delete token flows
2. api package — needs tests for API call methods using mock HTTP transport
3. cache package — needs behavioral tests for TTL, expiration, concurrent access
4. cmd package — focus on helper function tests (already started), avoid testing Cobra wiring

Quality guidelines

• Tests should verify behavior, not struct fields
• Each test should be able to catch a real bug
• Prefer fewer meaningful tests over many superficial ones
• Use the auth mock package as the quality template