[Improve] Measure initial Fast session response latency

[roomote-roomote]

​Opened on behalf of @daniel-lxs. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

Add a privacy-safe initial_human_turn classification to the existing fast_turn_settled event. The first canonical human prompt claims the marker atomically under the conversation lock; platform events, later prompts, concurrent turns, retries, and rollback-compatible history are handled without adding session or message identifiers.

First-response timing continues to start when the Fast turn enters the service and stops only after the first assistant reply is successfully delivered. Assistant-message persistence remains outside that latency boundary, and existing all-turn diagnostics remain intact.

Why this change was made

All-turn latency cannot answer how long a new Fast session takes to respond. Session paths and prompt kinds describe runtime state and retries, not whether a turn is the conversation's first human message, so a bounded explicit classification is required.

Impact

Operators can filter fast_turn_settled to initial_human_turn = true for initial Fast session latency without collecting conversation IDs, message IDs, prompts, or content. Classification is null when best-effort prompt persistence cannot establish it instead of guessing. Focused Fast-agent tests, package type checking, documentation validation, formatting, and the full pre-push suite pass.

[roomote-community]

No new code issues found. See task

• packages/cloud-agents/src/server/fast-agent/fast-agent-conversation-repository.ts:519 Preserve initial-turn classification for pre-existing platform events in rollback-compatible history.

_{Reviewed af41841}

[roomote-roomote]

Resolved the failed review finding in af418419: pre-existing rollback history now distinguishes strict platform-event envelopes from genuine human messages, preserving the first-human marker after upgrade. Added focused legacy-history coverage; 119 Fast-agent tests, package typecheck, formatting, and the full pre-push suite pass. See task