AI provider errors now surface the real message

When a call to an AI provider failed — bad API key, rate limit hit, model unavailable — the admin app sometimes displayed the literal string "true" as the error. Unhelpful does not begin to cover it.

The error pipeline now propagates the real provider message all the way through to the UI. If the provider says you exceeded your quota, you see "you exceeded your quota". If it says the model name is wrong, you see the model name. No more cryptic single-word errors.

If you have ever filed a support ticket because you saw true in the error box and had no idea what to do about it, this fix is for you.