Add service building MCP audit events

What does this MR do and why?

Add service building MCP audit events

Adds Mcp::Audit::CaptureEventService. It turns an MCP session or tool call into an AuditEvents::AiAuditEvent and hands it to the existing Ai::DuoWorkflows::ProcessAuditEventsWorker, so the ai_audit_events_storage_enabled gate, the ClickHouse/PostgreSQL write path, SIEM streaming, and audit_json.log are all reused rather than reimplemented.

Changelog: added EE: true

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