For symbols loaded from metadata, Roslyn exposes the MaybeNull/AllowNull flow attributes on the getter return value / setter value parameter instead of the property itself. SymbolAccessor only checked the property symbol, so cross-assembly source properties were treated as non-nullable, omitting the null guard and producing CS8601.

Add TryHasGetAttribute/TryHasSetAttribute helpers that also inspect the accessors and use them in IsReadNullable/IsWriteNullable.

Fixes #2192

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