Add comprehensive test suite

[davesleal]

Description

Build a comprehensive test suite to ensure reliability and enable confident contributions.

Proposed Tests

Unit Tests

• test_app_store_connect.py - ASC API client
• test_slack_handlers.py - Slack event handlers
• test_formatting.py - Message formatting
• test_session_management.py - Thread context

Integration Tests

• Mock Slack events and responses
• Mock App Store Connect API
• End-to-end command flows

Example

def test_format_review_for_slack():
    review = {
        "type": "review",
        "rating": 2,
        "title": "Crashes on launch",
        "body": "Broken app"
    }
    result = format_feedback_for_slack(review)
    
    assert "⭐⭐" in result
    assert "Crashes on launch" in result
    assert "Broken app" in result

Coverage Goals

• 80%+ code coverage
• All critical paths tested
• Edge cases handled

Test Infrastructure

• pytest for test framework
• pytest-cov for coverage
• Mock for external APIs
• Fixtures for test data

CI Integration

• Run tests on every PR
• Require tests for new features
• Track coverage trends