Update UnbrandedGeneratorVersion to 1.0.0-alpha.20260319.4

[Copilot]

Bumps the pinned @typespec/http-client-csharp unbranded emitter to 1.0.0-alpha.20260319.4 (TypeSpec commit d3b7f61) and aligns all dependent versions.

Changes

• eng/centralpackagemanagement/Directory.Generation.Packages.props — bump UnbrandedGeneratorVersion to 1.0.0-alpha.20260319.4
• eng/packages/http-client-csharp/package.json — bump @typespec/http-client-csharp to 1.0.0-alpha.20260319.4 and @azure-tools/typespec-client-generator-core to 0.66.3
• eng/packages/http-client-csharp/package-lock.json — regenerated to match updated dependencies
• eng/http-client-csharp-emitter-package.json / eng/http-client-csharp-emitter-package-lock.json — updated emitter artifact files
• Ran dotnet build /t:GenerateCode for all SCM-emitter libraries (Azure.AI.Extensions.OpenAI, Azure.AI.Projects, Azure.AI.Projects.Agents, Azure.AI.Speech.Transcription) — generated output is unchanged at this emitter version

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• Title of the pull request is clear and informative.
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• If an SDK is being regenerated based on a new swagger spec, a link to the pull request containing these swagger spec changes has been included above.
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