Fix 'Maximum update depth exceeded' on large TabbedForm submit

[imrans110]

Problem

Submitting a <TabbedForm> with many required fields throws "Maximum update depth exceeded" (React's NESTED_UPDATE_LIMIT). The same fields in a <SimpleForm> work fine. Reported in #11290 with a verified reproduction.

Root cause: useFormGroup subscribes to react-hook-form's validatingFields to compute a group-level isValidating that is not consumed anywhere (FormTabHeader and TranslatableInputsTab read only isValid). Because react-admin wraps every validator as async in useInput, react-hook-form toggles validatingFields per field on submit; subscribing to it re-renders every form group on each toggle, so a TabbedForm with enough required fields across tabs exceeds React's maximum update depth. SimpleForm has no FormGroup, so it is unaffected.

Solution

Stop subscribing to validatingFields in useFormGroup. The unused group-level isValidating becomes a constant false. Validation and the tab error badges — driven by errors / isValid — are unchanged. Per-input async validation spinners are unaffected, since those come from each input's own fieldState.isValidating (useController), not from useFormGroup.

How To Test

• Open the new ManyRequiredInputs story (ra-ui-materialui/forms/TabbedForm) in Storybook and click SAVE with the fields empty: on master it throws "Maximum update depth exceeded"; with this fix it validates normally.
• Run the regression unit test:

  yarn test-unit packages/ra-core/src/form/groups/useFormGroup.spec.tsx -t "validating state"

  It fails on master (the group re-renders ~38× on repeated re-validation) and passes with this fix.

The crash is timing-sensitive in a real browser; jsdom batches updates via act() and does not reproduce the depth overflow, so the unit test targets the underlying re-render behaviour while the Storybook story provides the in-browser check.

Additional Checks

• The PR targets master for a bugfix or a documentation fix, or next for a feature
• The PR includes unit tests (if not possible, describe why)
• The PR includes one or several stories (if not possible, describe why)
• The documentation is up to date (no change needed — the only isValidating reference in the docs uses react-hook-form's global useFormState().isValidating, which this change does not affect)

Closes #11290

[fzaninotto]

as seen in #11290, we need a reproduction first.