[cDAC][wasm] StackWalk_1 requires the Debugger contract, which WASM targets do not advertise #135034

Description

On WebAssembly targets, any cDAC IStackWalk walk that reaches a native-context frame throws:

ContractMissingException: Contract 'Debugger' is not advertised by the target.

This happens because StackWalk_1 calls IDebugger.GetHijackKind unconditionally for InitialNativeContext / NativeMarker states:

• src/native/managed/cdac/.../Contracts/StackWalk/StackWalk_1.cs — Next(), case StackWalkState.InitialNativeContext: case StackWalkState.NativeMarker: → _target.Contracts.Debugger.GetHijackKind(ip)
• same file, the x86 frame-pointer path (GetHijackKind(handle.Context.InstructionPointer))

But the data descriptor deliberately omits the contract on WASM:

// src/coreclr/vm/datadescriptor/datadescriptor.inc
#if defined(DEBUGGING_SUPPORTED) && !defined(TARGET_WASM)
CDAC_GLOBAL_CONTRACT(Debugger, c1)
#endif // DEBUGGING_SUPPORTED && !TARGET_WASM

On a browser-wasm target the default CreateStackWalk(threadData) seeds from the Frame chain (GetContextFromFrames). That produces an InitialNativeContext almost every time, so in practice every WASM walk fails on its first Next().

Repro

• Target: CoreCLR browser-wasm, nightly 12.0.0-alpha.1.26480.103 (runtime cd42bb5), published with UseMonoRuntime=false, RuntimeFlavor=CoreCLR.
• Reader: cDAC built from main @ bb1b237b389.
• Pause the page over CDP while managed code is running, then call IStackWalk.CreateStackWalk(threadData) and enumerate it.

The first frame (InitialNativeContext, ip 0x1, from an InlinedCallFrame) is yielded. The next MoveNext() then throws the ContractMissingException above.

Expected

WASM has no return-address hijacking: WasmFrameHandler.HandleHijackFrame throws PlatformNotSupportedException, and the runtime never installs hijacks there. The walker should treat a missing Debugger contract as HijackKind.None on targets that don't support hijacking, or gate the call on architecture or the contract's presence.

Notes

As a workaround, Blazor-Playground/nesm registers a version-"" fallback IDebugger for unadvertised targets. It returns HijackKind.None on WASM only and throws elsewhere. With that in place, walks complete on a live browser target.

Found while validating a live IStackWalk consumer in Blazor-Playground/nesm against CoreCLR browser-wasm; related to #130988.