Validate inline sparse payloads before dense allocation (#32863)
## Summary
- Validate embedded sparse values and indices before allocating a dense
destination.
- Reject unsupported sparse index element types before zero-size early
returns or allocation.
## Validation
- Warnings-as-errors build completed successfully.
- Focused sparse payload and index-type regression tests passed.
Co-authored-by: danielsongmicrosoft <danielsongmicrosoft@users.noreply.github.com>