Fix duplicate _start offset in Tensor.Resize

[prozolic]

Closes #128512

This PR fix the offset used when creating a ReadOnlySpan<T> that starts at _start in Tensor.Resize. AsTensorSpan()._reference already includes the _start offset, so applying it again via Unsafe.Add caused the span to start at the wrong position.

Changes:

• Remove Unsafe.Add when creating a ReadOnlySpan<T>
• Change from tensor.AsTensorSpan to Tensor.AsReadOnlyTensorSpan to match Tensor.ResizeTo
• Use (int)tensor.FlattenedLength instead of tensor._values.Length - tensor._start

[Copilot]

Pull request overview

Note

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Fixes a bug in Tensor.Resize where the source span incorrectly included data before the tensor's _start offset, causing wrong values to be copied when resizing a tensor created with a non-zero start.

Changes:

• Replaced manual _start-offset span creation with AsReadOnlyTensorSpan()._reference and used FlattenedLength as the span length.
• Added a new test TensorResizeWithStartTests covering resize cases for a tensor created with a non-zero start.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/Tensor.cs	Corrects source span reference and length for dense tensor resize.
src/libraries/System.Numerics.Tensors/tests/TensorTests.cs	Adds test coverage for Resize on tensors with a non-zero start.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.