chore: improve plan-based feature handling with plan hierarchy (#11335)

- Refactor HandleStripeEventService to better manage features by plan
- Add constants for features available in each plan tier (Startup,
Business, Enterprise)
- Add channel_instagram to Startup plan features
- Improve downgrade handling to properly disable higher-tier features
- Clean up and optimize tests for maintainability
- Add comprehensive test coverage for plan upgrades and downgrades

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Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

enterprise/app/views/fields/account_limits_field/_show.html.erb

@@ -0,0 +1,3 @@
+<% JSON.parse(field.to_s).each do |k,v| %>
+<%= k %>: <%= v %> </br>
+<% end %>