Surface model downrouting to Opus notifications in UI

[exedev-shelley]

Author: momentaryblip
Channel: #shelley
Link: https://discord.com/channels/1405685085923049482/1450334528210993295/1524824809094119435

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Issue

Users need visibility into when Shelley downgrades requests to Claude Opus due to rate limits or capacity constraints. Currently, there is no UI notification when this downrouting occurs, leaving users unaware of when their requests are being served by a different model than expected.

Context

From @momentaryblip:

I'm about to try out Fable, but wondering how potential down-routing to Opus happens in Shelley? Do you get a notification and need to agree or does it just happen (hopefully at least with a notification?) ?

Current State

According to @josharian:

Looks like we handle it correctly on the accounting side, but don't surface it in the UI anyway, which definitely seems worth fixing, since it appears this state of affairs isn't going to be short term(?)

[josharian]

One obvious approach is to insert a "model changed" warning entry into the conversation, the same way we do when LLM requests fail and are retried. We could do this any time a model changed for any reason (including intentional user actions).