Fix NullReferenceException for inherited ShouldSerialize methods

[haltandcatchwater]

Fixes #120561

Summary

ReflectionXmlSerializationWriter used BindingFlags.DeclaredOnly when looking up ShouldSerialize* methods at two call sites (attribute members loop and element/text members loop). This prevented finding methods declared on base classes, causing a NullReferenceException when serializing derived types.

The MemberMapping already stores the correctly resolved MethodInfo in CheckShouldPersistMethodInfo, resolved without DeclaredOnly during import. The IL-gen path (XmlSerializationWriterILGen) already uses this pre-resolved MethodInfo directly. This change aligns the reflection path to match.

Changes

• ReflectionXmlSerializationWriter.cs: Replace redundant GetMethod lookups with m.CheckShouldPersistMethodInfo at both call sites
• SerializationTypes.cs: Add BaseTypeWithShouldSerializeMethod and DerivedTypeWithInheritedShouldSerialize test types with both [XmlAttribute] and element properties
• XmlSerializerTests.cs: Add tests for inherited ShouldSerialize with default and non-default values, exercising both attribute and element serialization paths

[haltandcatchwater]

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[StephenMolloy]

This fix was already included as part of #126035. Closing, since that PR was merged.