feat: support children in ReferenceManyCount

[louzhedong]

Adds children support to ReferenceManyCountBase and forwards it through the MUI wrapper so callers can customize the total rendering with existing field components.

Implementation details:

• preserves the default raw total when no children are provided
• wraps custom children in a minimal RecordContextProvider with { id: 'count', total }
• keeps link/offline/error/loading behavior unchanged

Tests:

• yarn.cmd test-unit packages/ra-core/src/controller/field/ReferenceManyCountBase.spec.tsx --runInBand --watch=false
• yarn.cmd test-unit packages/ra-ui-materialui/src/field/ReferenceManyCount.spec.tsx --runInBand --watch=false
• yarn.cmd workspace ra-core build

[louzhedong]

Quick follow-up: I re-checked the branch locally and the targeted tests are still green on my side. The only failing GitHub checks are the Vercel deployment authorizations, which look external to the code change. If you'd like, I can re-run or narrow anything further once there's a maintainer review comment.

[fzaninotto]

Duplicate of #11266