EuiDataGrid can enter an infinite ResizeObserver loop and hang the tab when it has exactly one unsized column inside a size-animating ancestor

**Describe the bug**
EuiDataGrid can lock up the browser's main thread indefinitely when two conditions are both true:

1. The grid has exactly one column with no explicit initialWidth.
2. The grid is rendered inside an ancestor whose own size is also being measured/animated via ResizeObserver (e.g. an EuiAccordion that is currently expanding).

You can see it happening at kibana here https://github.com/elastic/kibana/issues/276400

**Steps to reproduce**

1. Render an EuiDataGrid with exactly one column and no initialWidth set on it.
2. Mount it inside an EuiAccordion (or any container that measures/animates its own size via ResizeObserver) at the moment the accordion opens.
3. Give the accordion enough width that the single stretched column's rendered width lands close to the scrollbar-visibility boundary (in practice this shows up more reliably at certain viewport widths, not all).
4. Observe: a brief visual flicker, then the tab becomes completely unresponsive.

**Expected behavior**

The grid should settle into a stable layout even with a single unsized column, without relying on a ResizeObserver feedback loop that has no termination condition.

**Impact and severity**
Very problematic for kibana ES|QL charts but we are mitigating https://github.com/elastic/kibana/pull/276423. Ideally I would like this to be solved at the eui side.

**Environment and versions**
kibana in main is using 116.5.0

Start by locating the EuiDataGrid resize and single-column sizing logic, then reproduce the issue with one unsized column inside an opening EuiAccordion at a viewport width near the scrollbar boundary. Done means the grid settles without a ResizeObserver loop or an unresponsive tab, with regression coverage for the reported conditions.

Written by the indexing model from the issue text.