Patch merge strategy fix

[KazaiMazai]

Patch merge strategy fix: generate patch merge strategy only for mutable optional properties

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Note

Auto-generated patch merge strategy now includes only mutable optional fields; docs updated accordingly.

• Macros:
  • Limit EntityModelMacro.patch generation to .patch(\.property) for attributes where isMutable == true.
  • Extend PropertyAttributes with isMutable: Bool and propagate it from VariableDeclSyntax via bindingSpecifier == var.
  • Update optional property collectors to attach isMutable and return only optional properties as before.
• Docs:
  • Clarify fragment merge strategy: generated for all mutable nullable properties.

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