fix: Fix client.push issue and update README for Slack client verification

[SumeetChougule]

Relates to: No specific issue linked.

Risks Low. The changes primarily involve bug fixes and documentation updates, which should not affect other parts of the system.

Background What does this PR do? This pull request fixes a critical issue in the client initialization process by addressing the clients.push error. It also updates the README for the Slack client to include instructions on verifying event subscriptions.

What kind of change is this? Bug fixes Documentation updates Documentation changes needed? My changes require a change to the project documentation. The README has been updated accordingly.

Testing Where should a reviewer start? Review the changes in agent/src/index.ts for the client initialization fix and the updated README.md in the packages/client-slack directory.

Detailed testing steps Verify that the client initialization process does not produce errors. Ensure the Slack client README includes the new section on event subscription verification.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

[odilitime]

going to make additional fixes after