🐛 fix(slack): member invite messages

[iamrajjoshi]

Slack notifications for member invites were not working

the problem was we were passing callback_id in the payload which has been deprecated since we switched to block-kit.
reference: https://api.slack.com/messaging/attachments-to-blocks#callback_id_replacement

we were also trying to send a message to user based on a simple response to the webhook payload which isn't how things work in block kit.

I updated the logic to use the Webhook client to send a notification.

Screen.Recording.2025-05-28.at.1.23.59.PM.mov

There are other places where we use callback_id,
I will make a linear ticket for folks to fix this during triage party based on my fix here

[codecov]

Codecov Report

Attention: Patch coverage is 93.93939% with 2 lines in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/sentry/integrations/slack/webhooks/action.py	87.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #92440      +/-   ##
==========================================
+ Coverage   86.55%   87.90%   +1.35%     
==========================================
  Files       10227    10227              
  Lines      586127   586083      -44     
  Branches    22763    22754       -9     
==========================================
+ Hits       507306   515186    +7880     
+ Misses      78392    70468    -7924     
  Partials      429      429