Fix XmlReader field name collision in DeserializationBenchmark

[Copilot]

internal XmlReader XmlReader field name collides with System.Xml.XmlReader type, causing XmlReader = XmlReader.Create(...) to fail compilation (compiler binds RHS XmlReader to the field, not the type).

Description

• Removed the XmlReader class field from DeserializationBenchmarks
• Replaced with a local using var _xmlReader = XmlReader.Create(...) inside XmlDictionaryDeserializer(), resolving the ambiguity and ensuring proper disposal per benchmark iteration

// Before (compile error)
internal XmlReader XmlReader;
XmlReader = XmlReader.Create(new StringReader(XmlData)); // RHS resolves to field, not type

// After
using var _xmlReader = XmlReader.Create(new StringReader(XmlData)); // unambiguous, disposed correctly

Related issues

Testing

Verified the Benchmarks project builds with 0 warnings and 0 errors.

FirelyTeam Checklist

• Update the title of the PR to be succinct and less than 50 characters
• Mark the PR with the label breaking change when this PR introduces breaking changes

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