Bug(PostHog AI): insight links shown to users do not always resolve

PostHog AI sometimes presents an insight it has just generated as if it can be opened from a URL, when in reality the insight is a transient artifact scoped to the conversation. The agent produces a markdown link of the form [Insight title (<id>)](/insights/<id>) in its reply, where <id> is the in-conversation artifact id and is not a real saved-insight short_id. Users click the link expecting to land on the insight and instead get nothing useful. This is a regression of the fix shipped in #44171.

How to reproduce

1. Open PostHog AI in any project (default mode is fine).
2. Ask the agent to build any chart, e.g. "show me average X by day of week as a bar chart".
3. In a follow-up turn, ask the agent for a link to the insight, or for the SQL.
4. The agent responds with a markdown link of the form [Title (<id>)](/insights/<id>) that does not resolve to a real insight.

Additional context

Observed in a real customer PostHog AI conversation on 2026-05-11, trace ID c6df86dd-8054-4b30-963f-6ac04a213848.

Reported in: https://posthoghelp.zendesk.com/agent/tickets/57991 (moved to PostHog: https://us.posthog.com/project/2/support/tickets/59328)

Reproduce this in PostHog AI by asking for a chart and then requesting an insight link or SQL. Trace the response path that generates the markdown link and verify that transient artifact IDs are not presented as saved-insight URLs; done means generated links resolve appropriately without breaking links to real saved insights.