Isolate concurrent Sentry requests by conversation

[dcramer]

It looks like concurrent Sentry-backed requests from different conversations can interfere with each other and cause one request to fail unexpectedly.

• Observed in one DM conversation while another request was active in a channel conversation.
• The likely trigger was two in-flight Sentry requests associated with different conversation contexts.
• This should be isolated per conversation so one request does not break another.
• Conversation ID for the failing request: slack:D0AS1V2KPRA:1781132981.551969