[bgen] Propagate nullability information for generic type arguments. Fixes #16860

[rolfbjarne]

Summary

When properties have generic delegate types (like Action<NSObject?, NSError?>), bgen now correctly propagates the ? nullability annotations on the generic type arguments in the generated C# code.

Previously, bgen only read the first byte of the [NullableAttribute(byte[])] array (for the outer type's nullability), ignoring the remaining bytes that encode nullability for generic type arguments. This caused properties like Action<UIViewController?, NSError?> to be rendered as Action<UIViewController, NSError> — losing the nullability annotations.

Changes

• src/bgen/AttributeManager.cs: Added GetNullabilityBytes() method that extracts the full byte array from [NullableAttribute].
• src/bgen/TypeManager.cs: Added FormatType(Type?, Type?, byte[]?) overload and a recursive FormatTypeUsedIn helper that consumes nullability bytes in depth-first traversal order, correctly skipping value types (which don't have bytes in the array).
• src/bgen/Generator.cs: Updated property rendering to pass nullability bytes. The "wrap" path applies nullability for all delegate types. The "non-wrap" path only applies for void-returning delegates (Action<>) to avoid Func<> covariance issues with trampoline signatures.
• tests/bgen/BGenTests.cs: Added GenericTypeNullability test with 10 assertions.
• tests/bgen/tests/generic-type-nullability.cs: Test input covering nullable args, non-nullable args, value types, many args, mixed patterns, and alternating nullability.

Key design decisions

1. Value types skip byte consumption: The C# compiler does NOT emit bytes for value types in the NullableAttribute array. The fix correctly identifies value types and skips them during traversal.
2. Func<> covariance guard: Trampolines use non-nullable generic args. This is fine for Action<in T> (contravariant) but breaks for Func<out T> (covariant). The non-wrap path only applies nullability for void-returning delegates.
3. Wrap path is unrestricted: The wrap path generates self-contained code without trampoline interaction, so it can safely apply nullability for any delegate type.

Fixes #16860

🤖 Pull request created by Copilot

[rolfbjarne]

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