Fix the formatting of the tensor ToString functions

[tannergooding]

The formatting was previously incorrect, resulting in output similar to:

System.Numerics.Tensors.Tensor<Single>[System.Numerics.Tensors.TensorShape]{
[
  [ 1, 2, ],
  [ 3, 4, ]]}

The correct formatting is meant to be:

System.Numerics.Tensors.Tensor<Int32>[2, 2] {
  [1, 2],
  [3, 4]
}

Explicit tests validating various scenarios were added.

[Copilot]

Pull Request Overview

This PR fixes the formatting of the ToString methods for tensor types to improve readability and provide better structured output. The previous implementation produced incorrectly formatted strings that were difficult to read, mixing type information with content formatting in an inconsistent way.

Key changes include:

• Refactored ToString implementation to centralize formatting logic across all tensor types
• Fixed shape representation to use comma-separated dimensions instead of TensorShape object representation
• Added proper indentation and structure for multi-dimensional tensor content display

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/Tensor_1.cs	Simplified ToString methods to delegate to centralized formatting logic
src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/TensorSpan_1.cs	Updated ToString methods to use centralized formatting with proper type name
src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/ReadOnlyTensorSpan_1.cs	Added comprehensive ToString overload with documentation and delegated to centralized logic
src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/Tensor.cs	Complete rewrite of ToString formatting logic with proper indentation and structure
src/libraries/System.Numerics.Tensors/ref/System.Numerics.Tensors.netcore.cs	Updated API surface to reflect new ToString method signatures
src/libraries/System.Numerics.Tensors/tests/TensorTests.cs	Added comprehensive tests for ToString functionality across different scenarios
src/libraries/System.Numerics.Tensors/tests/TensorSpanTests.cs	Added comprehensive tests for TensorSpan ToString functionality
src/libraries/System.Numerics.Tensors/tests/ReadOnlyTensorSpanTests.cs	Added comprehensive tests for ReadOnlyTensorSpan ToString functionality

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[jeffhandley]

@artl93 FYI - I'm merging this for RC1 as it meets the bug bar.