fix(ui): blocks field not respecting width styles in row layouts (#13502)

### What?

This PR applies `mergeFieldStyles` to the `BlocksField` component,
ensuring that custom admin styles such as `width` are correctly
respected when Blocks fields are placed inside row layouts.

### Why?

Previously, Blocks fields did not inherit or apply their `admin.width`
(or other merged field styles). For example, when placing two Blocks
fields side by side inside a row with `width: '50%'`, the widths were
ignored, causing layout issues.

### How?

- Imported and used `mergeFieldStyles` within `BlocksField`.
- Applied the merged styles to the root `<div>` via the `style` prop,
consistent with how other field components (like `TextField`) handle
styles.

Fixes #13498