[SRM] Deduplicate and optimize BlobWriterImpl.WriteConstant.

[teo-tsirpanis]

The logic of writing scalar constants was extracted to its own method that writes to a span. This method also got used by MetadataBuilder.GetOrAddConstantBlob, avoiding the use of pooled blob builders when writing scalar constants.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-reflection-metadata
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull Request Overview

This PR optimizes constant blob encoding by eliminating unnecessary allocations. It refactors the GetOrAddConstantBlob method and the WriteConstant methods to use stack-allocated buffers instead of pooled BlobBuilder instances for scalar (non-string) constants.

Key Changes:

• Introduced a new WriteScalarConstant method that writes scalar constants to a Span<byte> and returns the number of bytes written
• Refactored GetOrAddConstantBlob to use stack allocation for scalar values
• Updated both WriteConstant overloads to call WriteScalarConstant for non-string values

Reviewed Changes

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

File	Description
MetadataBuilder.Heaps.cs	Refactored GetOrAddConstantBlob to use stack-allocated buffer and new WriteScalarConstant method
BlobWriterImpl.cs	Added new WriteScalarConstant method and refactored existing WriteConstant overloads to use it

[Copilot]

The documentation should clarify that bytes must be at least sizeof(ulong) bytes in length to accommodate the largest scalar type. Additionally, it should document the expected behavior when value is null (returns 4 bytes for a zero uint).

Suggested change

	/// <param name="bytes">The span where the content will be encoded.</param>
	/// <param name="value">The constant value.</param>
	/// <param name="bytes">
	/// The span where the content will be encoded. Must be at least <c>sizeof(ulong)</c> bytes in length to accommodate the largest scalar type.
	/// </param>
	/// <param name="value">
	/// The constant value. If <paramref name="value"/> is <c>null</c>, writes 4 bytes representing a zero <c>uint</c>.
	/// </param>

[Copilot]

The stack allocation size sizeof(ulong) is duplicated across multiple call sites (lines 259, 272, and in MetadataBuilder.Heaps.cs line 259). Consider defining a named constant like MaxScalarConstantSize to make the code more maintainable and self-documenting.