test: add tests for suggester module (blackbox)

[ismaeldouglasdev]

Add missing test coverage for the suggester module.

The suggester.py handles LLM-powered conflict resolution suggestions. Currently has no test file.

Why

• Tests mock the anthropic API calls
• Verify correct prompt formatting
• Test error handling when API fails

Suggested tests

• test_suggestion_generation_with_valid_conflict
• test_suggestion_generation_api_error
• test_suggestion_prompt_formatting

[ismaeldouglasdev]

Hey! I'd like to work on this issue. Please assign me (@ismaeldouglasdev).