🔍 nit(slo): make slack message_limit_exceeded a halt (#84876)

<img width="767" alt="image"
src="https://github.com/user-attachments/assets/e9541c37-988a-4d52-9022-86c7c063a2b5"
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https://slack.com/help/articles/115002422943-Usage-limits-for-free-workspaces

resolves [SENTRY-3K4C](https://sentry.sentry.io/issues/6116162311/)

src/sentry/integrations/slack/utils/errors.py

@@ -19,6 +19,7 @@ class SlackSdkErrorCategory:
     MODAL_NOT_FOUND := SlackSdkErrorCategory("not_found"),
     RATE_LIMITED := SlackSdkErrorCategory("ratelimited"),
     RESTRICTED_ACTION := SlackSdkErrorCategory("restricted_action"),
+    MESSAGE_LIMIT_EXCEEDED := SlackSdkErrorCategory("message_limit_exceeded"),
 )
 
 """
@@ -30,6 +31,7 @@ class SlackSdkErrorCategory:
     CHANNEL_ARCHIVED,
     RATE_LIMITED,
     RESTRICTED_ACTION,
+    MESSAGE_LIMIT_EXCEEDED,
 )
 
 _CATEGORIES_BY_MESSAGE = {c.message: c for c in SLACK_SDK_ERROR_CATEGORIES}