Fix Tensor<T> slice pinning offset

[aw0lid]

Description

Tensor<T>.GetPinnableReference() returned a reference to the first element of the backing array without accounting for the slice's starting offset. As a result, pinning a non-zero-offset slice could expose the parent array's data instead of the slice's data.

Fix

Updated GetPinnableReference() to apply the tensor slice's _start offset using Unsafe.Add:

ret = ref Unsafe.Add(ref MemoryMarshal.GetArrayDataReference(_values), _start);

GetPinnedHandle() already uses GetPinnableReference(), so no changes were required there.

Tests

Added a regression test covering a non-zero-offset tensor slice. The test verifies that:

• The slice contains the expected values.
• GetPinnableReference() points to the first element of the slice.
• GetPinnedHandle() exposes the expected slice data.

closes #134691.

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