Update UnbrandedGeneratorVersion to 1.0.0-alpha.20260219.3

[Copilot]

Updates the TypeSpec HTTP Client C# generator from 1.0.0-alpha.20260219.2 to 1.0.0-alpha.20260219.3. Triggered by: microsoft/typespec@dd866a7

Description

• eng/Packages.Data.props — bumped UnbrandedGeneratorVersion
• eng/packages/http-client-csharp/package.json — bumped @typespec/http-client-csharp dependency; ran npm install to update lock file
• XmlSerializableVisitor.cs — updated to handle the generator's renamed private helper method (Write → WriteXml): the explicit IXmlSerializable.Write interface method remains unchanged and delegates to the renamed private WriteXml helper; introduced named constants for clarity:
  • IXmlSerializableWriteMethodName = nameof(IXmlSerializable.Write) — used for the explicit interface method signature and the WriteObjectValue switch case invocation
  • WriteMethodName = "WriteXml" — used for the body expression that calls the private helper
• XmlSerializableVisitorTests.cs + test data — updated assertions and expected output to reflect the new body expression (this.WriteXml(...))
• Regenerated all test projects via eng/packages/http-client-csharp/eng/scripts/Generate.ps1

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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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