feat(agent): echo initiating message_id on relay_message

[VojtechBartos]

Problem

When a turn finishes, the agent-server relays the final answer to the backend's relay_message endpoint with only text/text_parts. The relay carries no correlation to the user message it answers, so the backend has to guess who to @-mention in Slack from mutable state, which mis-tags replies in multiplayer threads.

Changes

• Echo the messageId of the user_message command that initiated the turn (the same id already used for duplicate-delivery suppression) as an optional message_id field in the relay_message POST body. The id is captured in the handler closure, so overlapping turns each relay their own initiating message's id.
• Backward compatible: the field is omitted whenever no message id is known (e.g. the boot prompt), so older backends see the exact same payload as before.

The consuming backend change (using message_id to attribute Slack replies) lands separately in PostHog/posthog, as a follow-up to the Slack follow-up queue work (PostHog/posthog#70806, PostHog/posthog#70762).

How did you test this?

• Added tests: relay echoes each turn's own initiating id end-to-end through the /command HTTP endpoint (including overlapping turns, the no-id case, and no id leaking from a failed turn into the next), relayAgentResponse pass-through, and the relay_message request body shape.
• pnpm --filter @posthog/agent test — 78 files, 1295 tests passed.
• pnpm --filter @posthog/agent typecheck and biome check on touched files — clean.

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit f11ab9f.}