test(prompts): expand coverage to 100%

- Add 51 new tests (18→69 total)
- Achieve 100% coverage: statements, branches, functions, lines
- Test RGB color handling in theme conversion
- Test all 5 theme names (socket, sunset, terracotta, lush, ultra)
- Add 18 wrapPrompt edge cases (errors, spinner, config validation)
- Add 4 createSeparator edge cases
- Add 3 complex Choice type scenarios
- Test isSocketTheme validation with 7 edge cases

Coverage improvements:
- Statements: 56.94% → 100%
- Branches: 43.18% → 100%
- Functions: 40% → 100%
- Lines: 56.94% → 100%

Author: jdalton