fix(auth,windows): avoid taking the address of a temporary provider

[SelaseKay]

Description

SignInWithProvider, LinkWithProvider, and ReauthenticateWithProvider passed &getProviderFromArguments(...), which takes the address of a temporary FederatedOAuthProvider. That is an MSVC extension (warning C4238). Flutter compiles Windows plugins with /W4 /WX, so the warning fails the build on newer MSVC toolsets.

Each call now stores the provider in a named local and passes that pointer. On desktop the C++ SDK only null-checks the pointer inside the call, so the local covers the use.

Our Windows CI already builds this file with Visual Studio 2026 18.9 / MSVC 14.51 and does not emit C4238. The report is from a development toolset, so that CI run will not show the warning disappearing.

Related Issues

Fixes #18698

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