Fix AV in ICorProfilerInfo2::GetClassLayout for Runtime Async continuation types

[UtkarshBandekar]

Description

ICorProfilerInfo2::GetClassLayout() crashes with an access violation when
called on a ClassID that refers to a dynamically-generated Runtime Async
Continuation MethodTable (introduced in #120411). These types have no
backing metadata (TypeDef/FieldDef tokens), but GetClassLayout assumes
every field it encounters can be resolved via FieldDesc::GetMemberDef().

For continuation types, ApproxFieldDescIterator::Next() can return a null
FieldDesc*, and the existing code calls GetMemberDef() on it
unconditionally, causing a null-pointer dereference at field.h:134
(return TokenFromRid(m_mb, mdtFieldDef)).

Fixes/relates to #120800.

Root cause

Confirmed via WinDbg on a local repro (.NET 11.0.0-preview.6.26359.118):

• !dumpmt /d <ClassID> resolves to Continuation_24_0_1_16_1, with
  mdToken: 0000000002000000 — RID 0, i.e. no real metadata token.
• The runtime already exposes MethodTable::IsContinuationWithoutMetadata()
  / TypeHandle::IsContinuationWithoutMetadata(), but GetClassLayout never
  calls it.
• Fault: mov ecx, dword ptr [rax+8] with rax == 0, inside
  FieldDesc::GetMemberDef inlined into
  ProfToEEInterfaceImpl::GetClassLayout+0x15e.
• Full native stack confirms this triggers from an entirely ordinary
  profiler heap-walk (ForceGC → GetClassLayout called per class), so any
  profiler doing a heap snapshot while a Runtime Async continuation is live
  will hit this.

Fix

Add an early check for typeHandle.IsContinuationWithoutMetadata() in
ProfToEEInterfaceImpl::GetClassLayout, returning CORPROF_E_DATAINCOMPLETE
— consistent with how this file already signals "no data available" in
other entrypoints (GetModuleInfo2, GetRVAStaticAddress,
EnumModuleFrozenObjects, etc.).

Repro

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net11.0</TargetFramework>
    <Features>runtime-async=on</Features>
  </PropertyGroup>
</Project>

Minimal repro: suspend an async method mid-await (object live across the
suspension point), then have a profiler call GetClassLayout on the
resulting continuation's ClassID — e.g. via a full-heap GC snapshot
(ForceGC + heap walk), which is what triggers it in practice.

Testing

• Add profiler test coverage under src/tests/profiler/ exercising
  GetClassLayout against a live Runtime Async continuation object.
• Verify CORPROF_E_DATAINCOMPLETE is returned rather than crashing.

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[UtkarshBandekar]

@dotnet-policy-service agree

[dotnet-policy-service]

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