feat: expose index in FormDataConsumer

[Aayush5154]

Fixes #10498

Problem

FormDataConsumer exposes formData and scopedFormData, but does not expose the index of the current item when used inside an ArrayInput.

Solution

Expose the current ArrayInput item index through FormDataConsumerRenderParams.

The index is extracted from the array source and passed to the render function.

How To Test

• Added a regression test for FormDataConsumer inside an ArrayInput.
• Verified that the index is correctly exposed for multiple array items.
• All test suites pass.

Additional Checks

• The PR targets next for a feature
• The PR includes unit tests
• The PR includes no unnecessary changes

[slax57]

This won't work for nested arrays (e.g. some.1.foo.2.bar). You should instead take the last match I believe.

[Aayush5154]

Updated to use matchAll and take the last match to handle nested arrays correctly. Also added a test for nested ArrayInput.

[fzaninotto]

Nice! But could you also update the doc to make this new feature discoverable? E.g. in https://marmelab.com/react-admin/Inputs.html#linking-two-inputs

[Aayush5154]

Nice! But could you also update the doc to make this new feature discoverable? E.g. in https://marmelab.com/react-admin/Inputs.html#linking-two-inputs

Thanks for the feedback! I've updated the documentation to make the new index parameter discoverable.