perf(http-client-csharp): parallelize Azure SDK regeneration

[JoshLove-msft]

Summary

• regenerate matching Azure SDK libraries concurrently instead of processing service directories serially
• install tsp-client and build the shared client plugin once before parallel generation
• keep ai libraries serialized to avoid conflicts in their shared plugin output
• preserve manual-run failure reporting for individual library failures

Validation

• built the C# emitter and generator
• ran Cop static analysis
• regenerated two Azure SDK libraries concurrently in a clean azure-sdk-for-net worktree

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/http-client-csharp@11284

commit: 957dd90

[azure-pipelines]

Azure Pipelines:
There may be pipelines that require an authorized user to comment /azp run to run.
Some pipeline(s) encountered errors during trigger evaluation.