Add tests for Slack listener handlers

[JiwaniZakir]

Problem

The test suite currently covers the research pipeline and utility functions, but the Slack listener handlers (member join, message, app_mention, commands) have no test coverage.

Proposed Solution

• Add tests for the key Slack event handlers
• Mock the Slack Bolt app object and verify handler behavior
• Test command parsing, permission checks, and response formatting
• Focus on the onboarding flow (member join → research trigger → intro generation)