Add some additional Tensor APIs

[tannergooding]

This resolves #117373, resolves #117372, resolves #117375

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[Copilot]

Pull Request Overview

This PR introduces new factory methods (CreateFromShape/CreateFromShapeUninitialized), implements span-based GetSpan and TryGetSpan APIs across Tensor, TensorSpan, and ReadOnlyTensorSpan, adds TensorMarshal interop helpers, and cleans up unused usings.

• Renamed static creation methods and updated all references and tests to CreateFromShape….
• Added GetSpan/TryGetSpan overloads (with nint and NIndex) and corresponding unit tests.
• Introduced TensorMarshal for creating tensor spans from raw memory.

Reviewed Changes

Copilot reviewed 59 out of 59 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/TensorTests.cs	Renamed factory calls, added GetSpan/TryGetSpan tests
tests/TensorSpanTests.cs	Added GetSpan/TryGetSpan tests for TensorSpan
src/.../Runtime/InteropServices/TensorMarshal.cs	New interop API for creating tensor spans
src/.../Tensor_1.cs & related files	Implemented GetSpan/TryGetSpan on Tensor, updated interface
src/.../ITensor_1.cs	Updated interface to declare new span-based methods

Comments suppressed due to low confidence (2)

src/libraries/System.Numerics.Tensors/tests/TensorTests.cs:2913

• Add test cases covering the GetSpan and TryGetSpan overloads that accept ReadOnlySpan<NIndex> to ensure the new NIndex-based APIs are exercised.

        public static void GetSpanTest()

src/libraries/System.Numerics.Tensors/src/System/Runtime/InteropServices/TensorMarshal.cs:21

• Consider validating that lengths.Length == strides.Length (or providing a default stride when empty) to avoid constructing a TensorSpan with mismatched dimension metadata.

        public static TensorSpan<T> CreateTensorSpan<T>(scoped ref T data, nint dataLength, scoped ReadOnlySpan<nint> lengths, scoped ReadOnlySpan<nint> strides, bool pinned)

[jeffhandley]

If this missed the Preview 7 snap, we should backport into the branch.

[tannergooding]

/ba-g unrelated wasm timeouts