MCP: XYChartConfig has no sort option — bar charts created via MCP are always ordered by category name

### Problem

`XYChartConfig` in `superset/mcp_service/chart/schemas.py` (line 1662 on master @ `7b351d5`) exposes no way to order categories. Its full field set is:

```
chart_type, x, y, kind, time_grain, orientation, stacked, group_by,
x_axis, y_axis, legend, x_axis_time_format, show_value, currency_format,
color_scheme, filters, row_limit, series_limit
```

There is no sort field, and `grep -ci sort` over the class body returns `0`.

Other chart configs in the same file do have one:

- `TableChartConfig` — `sort_by: List[str | SortByConfig]` (line 1572)
- `PieChartConfig` — `sort_by_metric: bool = True` (line 1047)

A `SortByConfig` model already exists (line 997) with `column` and `ascending` fields, and its docstring describes exactly the relevant use case:

```
Bare strings default to descending, which matches the
sort-by-metric "top N" pattern most commonly used for tables.
```

### Effect

Every `xy` chart created through MCP is emitted with:

```json
"x_axis_sort_series_type": "name",
"x_axis_sort_series_ascending": true
```

so categorical bar charts are always ordered by category name. For a ranking — the most common reason to reach for a bar chart — this is the wrong default and there is no way to override it through the tool. The equivalent is a one-click "Sort by metric" toggle in the Explore UI, so charts built via MCP start out worse than charts built by hand and need manual fixing afterwards.

Concrete case: a bar chart of orders per Brazilian state (27 categories, top value ~3x the second and ~900x the smallest) renders alphabetically AC, AL, AM, AP, BA… The concentration that motivates the chart is invisible until a human re-sorts it.

Sorting is also not reachable by other means, since `update_chart` accepts only the simplified `config` (raw `form_data` is not honoured), and unknown nested keys such as `x_axis: {"sort_by": ...}` are silently dropped rather than rejected (filed separately).

### Proposed change

Add a sort field to `XYChartConfig`, reusing the existing `SortByConfig`:

```python
sort_by: SortByConfig | None = Field(
None,
description="Order categories by a metric label or column. "
"Defaults to category name when unset.",
)
```

mapping to the native `x_axis_sort` / `x_axis_sort_asc` (or `x_axis_sort_series_type` / `x_axis_sort_series_ascending` for the series case) form_data keys.

Two things worth deciding in review:

1. **Whether to change the default.** Leaving it at category-name ordering is the compatible choice; defaulting bar charts to metric-descending matches what people usually want but would alter existing charts on regeneration. I'd lean toward keeping the current default and making sort opt-in.
2. **Scope.** `kind` covers `line`, `bar`, `area`, `scatter`. Sorting is meaningful for categorical `bar`/`area` but not for a temporal x-axis, so it may warrant a validator rejecting `sort_by` when `time_grain` is set.

Happy to implement whichever shape reviewers prefer.

### Environment

- Apache Superset 6.1.0 (docker), pydantic 2.11.7, fastmcp 3.4.5
- Field lists confirmed against `master` @ `7b351d5`