Wasm: full-framework R2R generic dictionary lookup leaves thread in preemptive GC mode #132022

When the whole framework (not just System.Private.CoreLib) is ReadyToRun-compiled
for wasm, several tests in the JIT subtree fail with a checked assert:

!"OBJECTREF being accessed while thread is in preemptive GC mode."
  src/coreclr/vm/object.cpp:514  Object::Validate

Affected tests: ProfileCastClassAndIsInst (+ _random1/2/3), Methodical_d1,
Methodical_r1, UnrollEqualsStartsWith -- all the same root cause.

These tests pass with interpreter or with only SPC R2R.

AI analysis below

The falure is in the R2R generic dictionary lookup slow path:

• R2R generic code hits a delay-load fixup READYTORUN_FIXUP_MethodDictionaryLookup.
• DynamicHelperFixup/DynamicHelperWorker (prestub.cpp) resolves it and builds
  DynamicHelper_GenericDictionaryLookup_Method_UseHelper (wasm stub,
  dynamichelpers.S). Both run and return cooperative (verified).
• The stub call_indirects g_pMethodWithSlotAndModule = managed
  GenericsHelpers.MethodWithSlotAndModule (in the R2R SPC), which calls the
  GenericHandleWorker [LibraryImport(QCall)] P/Invoke.

Instrumented trace (ProfileCastClassAndIsInst):

DynamicHelperWorker after-fixup: coopMode=1 (cooperative)
GenericHandleWorker enter:       coopMode=0 (preemptive - correct for a QCALL)
GenericHandleWorker exit:        coopMode=0 (still preemptive at C return)
ASSERT FAILED

The QCALL's coop -> preempt -> coop transition never restores COOPERATIVE on this
wasm path. After GenericHandleWorker returns, the R2R managed code resumes
PREEMPTIVE and asserts on the next OBJECTREF access.

With only the SPC R2R-compiled, the generic code runs interpreted and the
interpreter resolves dictionary lookups correctly.