Fixing 500 on slack OAuth

[sean-brydon]

What does this PR do?

Slack OAuth flow was sending a 500 when we hit callback URL.

The error received in log was error - uncaughtException: Error [ERR_STREAM_WRITE_AFTER_END]: write after end

After some time debugging it seems the handler didn't like us returning this as it was causing headers to be changed after the requests was sent.

Fixes #3327

Environment: Staging(main branch) / Production

Loom : https://www.loom.com/share/17e080a9a8fc479a9e7bf58ba7b020c1

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• I haven't added tests that prove my fix is effective or that my feature works

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[leog]

Good catch!