Add render prop support to ReferenceManyCount

[Will-thom]

Summary

• Allow ReferenceManyCountBase to render custom children using a record context containing total
• Forward ReferenceManyCount children so fields like NumberField source=total can format large counts
• Add stories, tests, and documentation for the new customization path

Fixes #11242

Tests

• node .yarn\releases\yarn-4.0.2.cjs test-unit packages/ra-core/src/controller/field/ReferenceManyCountBase.spec.tsx packages/ra-ui-materialui/src/field/ReferenceManyCount.spec.tsx
• node .yarn\releases\yarn-4.0.2.cjs exec eslint packages/ra-core/src/controller/field/ReferenceManyCountBase.tsx packages/ra-core/src/controller/field/ReferenceManyCountBase.stories.tsx packages/ra-core/src/controller/field/ReferenceManyCountBase.spec.tsx packages/ra-ui-materialui/src/field/ReferenceManyCount.tsx packages/ra-ui-materialui/src/field/ReferenceManyCount.stories.tsx packages/ra-ui-materialui/src/field/ReferenceManyCount.spec.tsx
• node .yarn\releases\yarn-4.0.2.cjs workspace ra-core build
• node .yarn\releases\yarn-4.0.2.cjs workspace ra-ui-materialui build

[fzaninotto]

Thanks for your PR. Now that I see the result, I think that supporting children is the wrong choice. Populating the RecordContext with a fake record containing only one total field feels very artificial, I don't find it intuitive and I don't find it shorter than using useReferenceManyFieldController directly.

I think that using a render function (as in RecordField) makes more sense, as there is no convention about any context.

<ReferenceManyCount
  reference="comments"
  target="post_id"
  render=({ total }) => new Intl.NumberFormat("de-DE").format(total)}
/>

If you want to use a Field component there, you can just pass the record explicitly (all fields compmonents support it):

<ReferenceManyCount
  reference="comments"
  target="post_id"
  render=({ total }) => <NumberField record={( total }} source="total" />}
/>

Besides, this syntax is already familiar as it's used by RecordField and DataTable.Col.

[Will-thom]

Thanks for the guidance. I reworked the PR to use a render function instead of children/RecordContext, matching the RecordField/DataTable.Col style you suggested. The updated implementation passes { total } directly to render, updates both ReferenceManyCount and ReferenceManyCountBase docs/stories, and adds focused coverage for the render customization path. I also updated the PR title to reflect the render prop approach.