Return turn attribution in completion notifications

[ccui-oai]

Summary

Adds per-turn attribution on the Codex server for skills, plugins, apps, and tools observed during a turn, and returns that attribution on TurnCompletedNotification.turn.attribution.

• accumulates attribution in AnalyticsEventsClient as skills/apps/plugins are tracked and tool items start
• drains the accumulated attribution when emitting completed/interrupted turn notifications
• adds protocol/schema/TypeScript definitions for TurnAttribution and related attribution entries

Testing

• cargo fmt
• cargo test -p codex-analytics records_turn_attribution_for_skill_plugin_app_and_tool
• cargo check -p codex-app-server
• cargo run -p codex-app-server-protocol --bin write_schema_fixtures --
• cargo test -p codex-app-server-protocol schema_fixtures

Enhancement request: internal analytics/UI need turn-level context for tool/plugin/skill/app usage.

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