#28973 Fix OOB pointer in TensorAt for sub-byte packed types

Description

Reject sub-byte packed types with appropriate error message. The TensorAt API returns a void*, it fundamentally cannot point to a sub-byte element. The fix is to reject sub-byte types at the API boundary, similar to how strings are rejected. Tests have been updated to reflect the changes.

Motivation and Context

TensorAt computed pointer offsets using logical element indices but Size() returns the byte size of the packed storage unit (1 byte for Int4x2/UInt4x2 which holds 2 elements). This caused out-of-bounds pointer returns when element indices exceeded the actual buffer size.

The API fundamentally cannot return a pointer to a sub-byte element, so reject sub-byte packed types with an appropriate error message, similar to how strings are already rejected.