Bug: Fluid typography edits leave stale size value in theme.json

When editing Custom Fluid Typography values in the Site Editor, the plugin correctly updates the fluid object (min and max) inside theme.json, but fails to update the corresponding size value. The size property retains its previous value after saving, resulting in an inconsistent, outdated typography definition.

Steps to Reproduce

1.	Go to Styles → Typography and edit a Custom Fluid typography size.
2.	Change both Minimum and Maximum sizes.
3.	Click Save to update the theme via Create Block Theme.
4.	Check the generated theme.json.

Actual Behavior

In theme.json, only the fluid values are updated. Even after setting 5.63rem in the Editor, the size value remains "3rem" (or whatever it was before the change).

"fontSizes": [
  {
    "slug": "custom-fluid",
    "name": "Custom Fluid",
    "fluid": {
      "min": "1.5rem",
      "max": "5.63rem"  // updated correctly
    },
    "size": "3rem"      // ❌ still old value
  }
]

Expected Behavior

The size property should be kept in sync with the user’s chosen Maximum size, just like other typography presets created or edited via the Editor. The size should never remain stale once a user updates a custom fluid size.

Reproduce the issue in the Site Editor by changing both Custom Fluid Typography limits, saving through Create Block Theme, and inspecting the generated theme.json. Trace the save path that updates the fluid object, then verify that the size property also matches the chosen Maximum size and no longer retains its previous value.