[unified_analytics] Default agent to AiAgent.detectAgentName #2671

Analytics(...) and Analytics.development(...) default agent to null instead of calling AiAgent.detectAgentName(io.Platform.environment), leaving ai_agent unset unless each consumer wires it manually.

• Impact: Callers that do not explicitly pass agent:—such as analysis_server (dart-lang/sdk#64591), dtd_impl, and dart_mcp_server—emit ai_agent = null on all events even when launched by an AI coding agent.

Proposed Fix

• Default agent: agent ?? AiAgent.detectAgentName(io.Platform.environment) in the production Analytics(...) and Analytics.development(...) factories, while leaving Analytics.fake(...) isolated from io.Platform.environment so unit tests stay hermetic when run inside an AI agent.