Add audit events for AI Catalog MCP server create and update

What does this MR do and why?

Related to: Enable custom agents to access external data vi... (#590708)

Send audit events when an MCP server is created or updated, following the same pattern as the agents audit events. Introduces a shared BaseService to deduplicate the send_audit_events logic across CreateService and UpdateService, and adds an AuditEventMessageService with descriptive change messages for name, URL, description, transport, and auth type changes.