test: make allocation assertion deterministic

[thomhurst]

Summary

• Measure wrapper construction with GC.GetAllocatedBytesForCurrentThread() instead of process-wide live heap size.
• Cache the task factory before measurement so the delta covers only the wrapper array and completion sources.
• Retain the existing value-type assertions that detect a regression from structs to heap-allocated wrapper objects.

Root cause

GC.GetTotalMemory(false) includes unrelated allocations from concurrently running TUnit tests and target frameworks. That made the fixed 200 KB threshold nondeterministic even when wrapper behavior was unchanged.

Test plan

• Two consecutive dotnet test TomLonghurst.EnumerableAsyncProcessor.sln -c Release passes
• 1,074/1,074 tests passed per run across concurrent net8.0 and net9.0 execution
• git diff --check

Closes #346

[greptile-apps]

Greptile Summary

This PR makes the wrapper allocation test deterministic. The main changes are:

• Replaces process-wide live-memory measurement with per-thread allocated-byte counters.
• Creates the shared task factory before the measured region.
• Keeps the existing allocation threshold and value-type checks.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• Both allocation snapshots occur in the same uninterrupted synchronous region.
• The cached delegate matches the test's goal of measuring wrapper storage and completion sources.

Important Files Changed

Filename	Overview
EnumerableAsyncProcessor.UnitTests/TaskWrapperStructValidationTests.cs	Uses thread-local allocation accounting and excludes task-factory creation from the measured wrapper-construction region.

_{Reviews (1): Last reviewed commit: "test: isolate wrapper allocation measure..." | Re-trigger Greptile}