Annotate Windows interop declarations with [SupportedOSPlatform("windows")]

[Copilot]

Annotates the platform-specific DllImport / LibraryImport / UnmanagedCallersOnly declarations under src/libraries/Common/src/Interop/Windows/** with [SupportedOSPlatform("windows")], and guards their callers.

This lets the wasm CoreCLR P/Invoke scan (PInvokeCollector) skip these declarations on browser/wasi — removing WASM0066 noise at the source — and makes them visible to CA1416 platform-compatibility analysis.

Contributes to #132776

Scope

• Windows interop only — all 38 modules under src/libraries/Common/src/Interop/Windows/. Unix/macOS interop is not part of this change.
• The attribute is guarded with #if NET so the shared files still compile for netstandard/netfx consumers (e.g. TestUtilities on net481), where SupportedOSPlatformAttribute does not exist.

Approach

• Granularity. Type-level (coarse) annotation on the Interop.<Module> partial class where it is safe. Per-method annotation is used where a module's const/struct/enum members are consumed by cross-platform code — e.g. the public enums in System.Net.Primitives (SslProtocols, SocketError, …) whose values derive from Interop.SChannel.* / Interop.Winsock.* / Interop.Crypt32.* — so those members stay platform-neutral. Kernel32, Advapi32, Crypt32, BCrypt, and WinSock are per-method; the remaining modules are type-level.
• Callers. Cross-platform files in System.Private.CoreLib reach Windows-only interop only through compile-time gates (#if TARGET_WINDOWS, #if FEATURE_COMINTEROP, *.Windows.cs, *.Nls.cs) that CA1416 cannot observe; those files carry a file-level #pragma warning disable CA1416. Managed wrapper helpers that call an annotated P/Invoke are themselves annotated [SupportedOSPlatform("windows")].

Validation

• System.Private.CoreLib (Windows) builds clean.
• Cross-platform consumers — System.Net.*, System.Security.Cryptography, System.Diagnostics.*, System.IO.*, System.Console, System.DirectoryServices.Protocols, and others — build clean across all target frameworks, including browser, wasi, unix, linux, osx, and android.
• netstandard/net481 consumers of the shared interop build clean.

Note

This pull request was authored with GitHub Copilot.

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