Expose Metabot step-limit continuation in useMetabot

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## Title

Expose Metabot step-limit continuation in `useMetabot`

## Description of the bug

When Metabot reaches its step limit, `` displays the error state and **Continue** button. A custom Metabot rendered with `useMetabot` exposes neither this UI nor the data needed to render it.


## Reproduction steps

1. Temporarily lower the maximum iteration to `2` in [profiles.clj](), matching the reproduction in metabase/metabase#79860.
2. Render `` with “year-over-year order count by product”. The **Continue** button appears normally.
3. Render a custom Metabot with `useMetabot`. The error state and **Continue** button are not exposed, and no `useMetabot` data is available to wire them up.

## Expected behavior

`useMetabot` should expose the step-limit error state and data required to render the continuation UI, consistent with ``.

## Logs

None observed.

## Diagnostic information

Tested at `9a675a4749fd66158e08d15aa9c4aca7205b3f85`. Code verification indicates the issue should also occur on `master` at `3989cb5eb38481d980888a58fbc374a4092a8c89`.

## Severity

P2

Start by comparing the `useMetabot` entry point with the `` behavior, using the step-limit reproduction and the maximum-iteration setting in `src/metabase/metabot/agent/profiles.clj`. Trace how the error state and Continue action are represented, then verify that a custom Metabot can access the required state and data to render equivalent continuation UI.

Written by the indexing model from the issue text.