Investigate shared restore races in parallel C# SDK regeneration

[Copilot]

Parallel SDK regeneration can restore shared projects such as Azure.SdkAnalyzers into the same intermediate directory, causing file conflicts and skipping subsequent batches.

• Findings: Existing concurrency fixes isolate plugin builds and coordinate npm installation. The outer SDK restores still share intermediate paths, with no retry before skipping later batches.
• Changes: None. This investigation checked whether the issue was already addressed; the inspected code indicates it remains unresolved.

• Fixes [http-client-csharp] Parallel Azure SDK regeneration races on shared MSBuild intermediate files #11822

[azure-pipelines]

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