Add dedicated unit tests for sanitizeNextPath

[ryota-murakami]

Summary

sanitizeNextPath lacks dedicated unit tests (especially the null-input branch).

Source

P2 follow-up from /ship adversarial review of PR #176 (silent GitHub token refresh).

Details

Currently exercised only indirectly via the refresh-route tests. Direct coverage missing.

Acceptance

• Test file src/tests/unit/lib/utils/sanitize-next.test.ts created
• Coverage: null input, relative paths, absolute paths, javascript: URLs, protocol-relative URLs, double slashes, fallback default