Update UnbrandedGeneratorVersion to 1.0.0-alpha.20260306.2

[Copilot]

Bumps @typespec/http-client-csharp and UnbrandedGeneratorVersion to 1.0.0-alpha.20260306.2. Key behavioral change: XmlModelWriteCore visibility changed from protected to internal virtual.

Description

• Updated eng/centralpackagemanagement/Directory.Generation.Packages.props UnbrandedGeneratorVersion
• Updated eng/packages/http-client-csharp/package.json dependency version and refreshed package-lock.json
• Regenerated TestProjects/Local/Basic-TypeSpec — all XML serialization models now emit internal virtual void XmlModelWriteCore(...) / internal override void XmlModelWriteCore(...)
• Validated all unit tests pass (194 Azure.Generator, 8 Local, 115 Mgmt); no testdata files required updates as no existing tests directly asserted XmlModelWriteCore visibility

TypeSpec commit: microsoft/typespec@c7eaf96

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