feat(api): Add parameter to skip conversation continuity emails on a per-message basis

Is your feature request related to a problem? Please describe. Yes. When sending a conversation reply via the REST API (POST /api/v1/accounts/{account_id}/conversations/{conversation_id}/messages), Chatwoot automatically triggers an outbound "conversation continuity" email to the customer if they have an email address on file and are marked offline. Currently, there is no way to suppress this email on an individual message level. The only workarounds are to globally disable the email continuity feature at the account level via the Rails console, or perform a complex "Delete-Post-Restore" sequence by temporarily stripping the customer's email address via the API before sending the message. This introduces race conditions in high-concurrency environments and complicates automated workflows (e.g., sending silent diagnostic data, automated receipts, or system-triggered notes that customers shouldn't be emailed about). Describe the solution you'd like …