Extract wasm string-ctor thunk selection and prestub native-helper fallback from next-blocker-spc

[Copilot]

Extracts the requested subset from main...AndyAyersMS:runtime:next-blocker-spc: the GetCookieForCalliSig string-constructor handling and the prestub.cpp managed-FCall helper entrypoint fix. This narrows the PR to the two runtime paths needed to unblock wasm mixed interpreter/R2R dispatch.

• src/coreclr/vm/wasm/helpers.cpp — GetCookieForCalliSig

  • Adds a string-constructor special case so interpreter→R2R calli cookies use the constructor factory thunk shape (matching existing R2R→interpreter string-ctor handling).
  • Uses explicit thunk-key mapping by constructor arity, including ReadOnlySpan<char> constructor handling.

• src/coreclr/vm/prestub.cpp — managed FCall helper publication

  • Handles both managed-helper forms under portable entrypoints:
    • interpreter bytecode available → publish interpreter data;
    • interpreter bytecode absent (native R2R helper) → publish helper native actual code.
  • Prevents routing back into prestub when the helper resolves to native code.

if (ilStubInterpData != NULL)
{
    SetInterpreterCode((InterpByteCodeStart*)ilStubInterpData);
    PortableEntryPoint::SetInterpreterData(entryPoint, (PCODE)(TADDR)ilStubInterpData);
}
else
{
    _ASSERTE(PortableEntryPoint::HasNativeEntryPoint(pCode));
    PortableEntryPoint::SetActualCode(entryPoint, (PCODE)(TADDR)PortableEntryPoint::GetActualCode(pCode));
}

[Copilot]

Copilot was unable to review this pull request because there is no eligible user to bill. To allow Copilot reviews on bot-authored pull requests, enable direct organization billing in your organization's Copilot settings.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[dotnet-policy-service]

Tagging subscribers to this area: @JulieLeeMSFT, @BrzVlad, @janvorli
See info in area-owners.md if you want to be subscribed.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[davidwrighton]

/ba-g This change does not affect the iOS build in any way.