Reduce allocations in LazyItemEvaluator.ProcessMetadataElements by avoiding enumerator boxing

[nareshjo]

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• Issue: ProcessMetadataElements(ProjectItemElement, OperationBuilderWithMetadata) calls AddRange(itemElement.Metadata) on ImmutableArray.Builder. ETW traces show this method as an allocation hotspot due to allocations from IEnumerable.GetEnumerator interface calls during AddRange(KeyValuePair<K, V>[]), indicating the collection's struct enumerator is being boxed to satisfy the IEnumerable parameter requirement.
• Issue type: AVOID boxing enumerators of built-in types
• Proposed fix: Remove the AddRange(KeyValuePair<K, V>[]) call and add elements individually inside the existing ForEach(Action) loop. The C# compiler optimizes ForEach(Action) to directly use the collection's concrete GetEnumerator() method, avoiding boxing when a struct enumerator is available. This also eliminates duplicate enumeration of Metadata, providing two performance benefits:

1. Eliminates boxing allocation - The struct enumerator is used directly without boxing to the IEnumerable interface
2. Eliminates duplicate enumeration - The collection is only enumerated once instead of twice

Best practices wiki
See related failure in PRISM
ADO work item

[Copilot]

Pull request overview

This PR optimizes ProcessMetadataElements to reduce allocations by eliminating enumerator boxing. The change moves metadata element addition from a single AddRange call to individual Add calls within the existing loop.

Key changes:

• Removed AddRange(itemElement.Metadata) call that caused boxing of struct enumerator
• Added individual Add(metadatumElement) calls inside the existing foreach loop
• Eliminated duplicate enumeration of the Metadata collection

[nareshjo]

@rainersigwald thanks for reviewing. I don't have permissions to merge. Could you please merge it if you do.

[DustinCampbell]

I don't know that I have merge permissions either, but I've spent a lot of time in LazyItemEvaluator lately, and this looks good to me.

[rainersigwald]

We'll merge after an insertion with #12950 validates that we haven't accidentally regressed VS while we were missing coverage.