[mono] Fix interface call in case of reabstraction

[BrzVlad]

An interface is assigned an offset in the vtable of a class that implements it, so that a call through a method on that interface will do a call through the vtable slot. There are multiple places where this offset computation happens, for example mono_class_setup_count_virtual_methods. Method reabstraction doesn't create a new slot.

Interface calls are dispatched through imt slots that are placed before the vtable. A slot is initialized in build_imt_slots so that it calls through the corresponding vtable slot. For slots where we have collision of multiple interface methods, we emit some thunk that does repeated checks to dispatch to the right method. The build_imt_slots method didn't handle reabstraction so the vtable slot to be called for an interface method was incorrectly computed. This would appear to suggest that all cases of interface method reabstraction are broken. However, it wasn't the case because even though we incorrectly initialize the imt_slot with the wrong implementing method, mini_resolve_imt_method will end up patching the imt_slot if we don't have collisions, over-writting with the correct target.

Added a testcase with multiple reabstracted methods on a type.

Fixes #129496

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[Copilot]

Pull request overview

This PR updates Mono’s IMT slot construction to ignore reabstracted interface methods when computing the vtable slot to call, preventing incorrect interface dispatch for IMT slots that require collision trampolines. It also adds a regression test that defines an interface with many reabstracted members to validate dispatch stability.

Changes:

• Skip reabstracted interface methods in build_imt_slots (both the generic-interface fast path and the normal path) so they don’t affect vt_slot accounting or IMT entry construction.
• Add a new regression test project and test case exercising interface dispatch in the presence of many reabstracted members.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/mono/mono/metadata/object.c	Skips reabstracted methods while building IMT entries so vtable slot selection remains correct, especially for collision trampolines.
src/tests/Loader/classloader/DefaultInterfaceMethods/regressions/ReabstractedInterfaceMethodIMT.cs	Adds a regression test that invokes many interface methods on a type whose interface includes many reabstracted members.
src/tests/Loader/classloader/DefaultInterfaceMethods/regressions/ReabstractedInterfaceMethodIMT.csproj	Adds the test project file for building/running the new regression test.