Reduce Helix work items with platform-aware partitioning

[MichaelSimons]

Summary

Consolidate Helix work item partitions to target ~5 minutes per work item, reducing overall work item count by ~58%.

Problem

The current base method limit of 16 methods per shard was producing far too many short-running work items, especially on faster platforms like mac-arm64 where the average work item took only 42 seconds.

Changes

All partitioning configuration now lives in test/UnitTests.proj:

• Raised base method limit from 16 → 32 (64 for FullFramework, 128 for osx-arm64)
• Moved all limit configuration into UnitTests.proj — the C# task (SDKCustomCreateXUnitWorkItemsWithTestExclusion) is now a consumer of BaseMethodLimit rather than hardcoding values
• Formula: effective limit = BaseMethodLimit × MethodLimitMultiplier

Projected Impact

Queue	Current Avg	Projected Avg	Work Items
Windows	2.9 min	~5.7 min	590 → ~295
mac-x64	2.2 min	~4.5 min	352 → ~176
Linux	1.9 min	~3.8 min	347 → ~174
mac-arm64	0.7 min	~5.6 min	347 → ~44
Total			1,636 → ~689 (~58% reduction)

Configuration at a glance (test/UnitTests.proj)

<!-- Base method limit: platform-aware -->
<BaseMethodLimit Condition="'$(TestFullMSBuild)' == 'true'">64</BaseMethodLimit>
<BaseMethodLimit Condition="$(TargetRid.StartsWith('osx-arm64'))">128</BaseMethodLimit>
<BaseMethodLimit Condition="'$(BaseMethodLimit)' == ''">32</BaseMethodLimit>

<!-- Per-project multipliers -->
<!-- NetAnalyzers:      5x  (effective: 160 / 640 on arm64) -->
<!-- TemplateEngine:   20x  (effective: 640 - prevents splitting) -->
<!-- ApiCompat/GenAPI: 10x  (effective: 320 - prevents splitting) -->

[Copilot]

Pull request overview

This PR centralizes Helix xUnit work-item partition sizing in test/UnitTests.proj and makes partitioning platform-aware to reduce short-running Helix shards.

Changes:

• Adds BaseMethodLimit configuration with larger defaults for FullFramework and osx-arm64 legs.
• Passes BaseMethodLimit into the custom xUnit Helix work-item task.
• Updates the task to consume the configured base limit and apply per-project multipliers.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/UnitTests.proj	Defines platform-aware base method limits for Helix test partitioning.
test/xunit-runner/XUnitRunner.targets	Passes the configured base method limit into the Helix work-item creation task.
test/HelixTasks/SDKCustomCreateXUnitWorkItemsWithTestExclusion.cs	Uses BaseMethodLimit instead of hardcoded task-local defaults when scheduling test shards.

[chcosta]

musical-lamp?

[MichaelSimons]

musical-lamp?

Compliments of the github copilot app!

[MichaelSimons]

Mac x64 hung but the rest of the affect legs completed. There is some additional fine tuning to do but I am of the mindset to get this in asap to get some immediate gains and see the impact to the overall system.

Here are the actual workitem impact of this PR. The PR descriptions was theoretical in the sense that test assemblies are the granularity limit.

Force merging ahead of a full PR validation run.