GeneratePropertyBag code generation fails when "?" nullable is used with the "partial" class modifier

How to reproduce:
1. Open the project “IN-111253_serialization-repro”
2. Observe the Console

Expected result: No Unity.Properties.SourceGenerator errors in Console
Actual result: Unity.Properties.SourceGenerator errors in Console

Reproducible with: 6000.0.0b12, 6000.0.55f1, 6000.2.0f1 (eed1c594c913), 6000.3.0a4
Not reproducible with: 6000.0.0b11 (a707ca4efec4)

Reproducible on: Windows 11, macOS 15.5 (M3)
Not reproducible on: No other environments tested

Notes:

• The error can be fixed by using exclusively the “partial” class modifier or removing the “?” nullable. This can be done in the TestScript.cs file
• Errors:
  • {{Unity.Properties.SourceGenerator\Unity.Properties.SourceGenerator.PropertyBagGenerator\Bar_PropertyBag.cs(1,18): error CS1514:
    Unknown macro: { expected}

    }

  • Unity.Properties.SourceGenerator\Unity.Properties.SourceGenerator.PropertyBagGenerator\Bar_PropertyBag.cs(1,18): error CS1513: } expected
  • Unity.Properties.SourceGenerator\Unity.Properties.SourceGenerator.PropertyBagGenerator\Bar_PropertyBag.cs(1,18): error CS1031: Type expected
  • Unity.Properties.SourceGenerator\Unity.Properties.SourceGenerator.PropertyBagGenerator\Bar_PropertyBag.cs(2,1): error CS8803: Top-level statements must precede namespace and type declarations.
  • Unity.Properties.SourceGenerator\Unity.Properties.SourceGenerator.PropertyBagGenerator\Bar_PropertyBag.cs(3,5): error CS0106: The modifier 'internal' is not valid for this item
  • Unity.Properties.SourceGenerator\Unity.Properties.SourceGenerator.PropertyBagGenerator\Bar_PropertyBag.cs(8,64): error CS1525: Invalid expression term 'sealed'
  • Unity.Properties.SourceGenerator\Unity.Properties.SourceGenerator.PropertyBagGenerator\Bar_PropertyBag.cs(8,64): error CS1002: ; expected