Fix | SqlVector: Explicitly perform little-endian multibyte writes

[edwardneal]

Description

This deals with a minor point which came out of the original implementation PR: when we build (or read) the byte array representing a vector({size}, float32), we now explicitly do so using BinaryPrimitives' little-endian methods.

There are two slightly unusual points:

• netfx doesn't have the BinaryPrimitives.WriteSingleLittleEndian method. Instead, we fall back to the existing BitConverterCompatible.SingleToInt32Bits on this target.
• When building the byte array, we perform two casts: (float)(object)valueSpan[i]. This is another variation of the same pattern used elsewhere of (T)(object)item, and the same pattern holds: the JIT sees that valueSpan is actually a ReadOnlySpan<float> and eliminates the redundant cast. An example of this on Sharplab is here.

Issues

Fixes #3790.

Testing

Automated tests continue to pass.

[apoorvdeshmukh]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[Copilot]

Pull request overview

This PR refactors the SqlVector<T> implementation to use explicit little-endian byte operations when serializing and deserializing vector data. The change replaces platform-specific memory marshaling/copying approaches with consistent loop-based operations using BinaryPrimitives methods to ensure correct endianness.

Key Changes:

• Replaced manual bit manipulation and platform-specific Buffer.BlockCopy/MemoryMarshal operations with explicit little-endian read/write methods
• Introduced platform-specific handling: BinaryPrimitives.WriteSingleLittleEndian for .NET and BitConverterCompatible.SingleToInt32Bits + BinaryPrimitives.WriteInt32LittleEndian for .NET Framework
• Ensured symmetry between serialization (MakeTdsBytes) and deserialization (MakeArray) operations

[Copilot]

The conditional check for typeof(T) == typeof(float) should include an else clause that throws an exception for unsupported types. Currently, if T is not float, the method would return a byte array with only the header populated and no element data. While GetTypeFieldsOrThrow() prevents non-float types at construction time, adding an explicit else clause here would make the code more defensive and maintainable for future type additions. Consider adding: else { throw SQL.VectorTypeNotSupported(typeof(T).FullName); }

[Copilot]

The conditional check for typeof(T) == typeof(float) should include an else clause that throws an exception for unsupported types. Currently, if T is not float, the method would return an uninitialized array. While GetTypeFieldsOrThrow() prevents non-float types at construction time, adding an explicit else clause here would make the code more defensive and maintainable for future type additions. Consider adding: else { throw SQL.VectorTypeNotSupported(typeof(T).FullName); }

Suggested change

	}
	}
	else
	{
	throw SQL.VectorTypeNotSupported(typeof(T).FullName);
	}