[release/9.0-staging][mono][gc] Fix gc descriptor computation for InlineArray structs

[BrzVlad]

compute_class_bitmap iterates over all ref field slots in the current class so we can produce a GC descriptor. field_iter represents how many times the type in question is repeated in the current struct. Instead of bumping the current offset by the size of the repeated field, for each iteration, we were adding field_offset which is wrong.

Customer Impact

• Customer reported
• Found internally

Types having InlineArray attribute are not correctly scanned by the GC for refs. This can lead to GC crashes on Maui applications. User reported problem migrating from legacy xamarin to maui Humanizr/Humanizer#1572.

Regression

• Yes
• No

InlineArray attribute for types was added around .NET8, with some uses showing up in the libraries code in .NET9, which could lead to regressions for some users.

Testing

Tested on local test that the GC descriptor is now computed correctly for types with InlineArray attribute. Verified fix on sample app provided by customer.

Risk

Low. The fix is localized to types that have InlineArray attribute, where the previous implementation was completely broken.

[Copilot]

Pull Request Overview

This PR corrects the GC bitmap generation for structs marked with the InlineArray attribute by using the actual element size when iterating through array slots rather than repeatedly adding the field’s original offset.

• Introduced field_size to hold the size of each element via mono_type_size
• Wrapped the inline‐array logic in braces to scope assignments
• Changed field_instance_offset to increment by field_size instead of field_offset

Comments suppressed due to low confidence (1)

src/mono/mono/metadata/object.c:914

• [nitpick] The name field_size is ambiguous in this context—consider renaming it to element_size to clarify that it represents the size of each array element.

				field_size = mono_type_size (field->type, &align);

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