[release/10.0] Add ERROR_NETNAME_DELETED to IsPathUnreachableError for Windows localhost removal (#124869)

Backport of #121569 to release/10.0

/cc @jozkee @Copilot

## Customer Impact

- [ ] Customer reported
- [x] Found internally

This only affects `ResolveLinkTarget(returnFinalTarget: true)`. Code
first tries to open the final file via `OpenSafeFileHandle` as an
optimized approach, and when that fails it checks IsPathUnreachableError
to decide whether to fall back to manually traversal with
`DeviceIoControl`. Since `ERROR_NETNAME_DELETED` was not being
categorized as a path unreachable error, the fallback never happens and
an `IOException` is thrown instead.

We don't have customer reports yet, but we expect them to come
eventually, and this is technically a `regression-from-last-release`.

## Regression

- [x] Yes
- [ ] No

This is a regression caused by a Windows OS update that removed
localhost support. The error started appearing in CI builds around
October 2025.


## Testing

The original PR #121569 was merged to main on November 13, 2025 and
resolved the CI failures.

## Risk

Low. The change adds a single error code constant (ERROR_NETNAME_DELETED
= 0x40) and includes it in the existing `IsPathUnreachableError` pattern
match alongside other similar network path error codes
(ERROR_BAD_NETPATH, ERROR_BAD_NET_NAME, ERROR_NETWORK_ACCESS_DENIED,
etc.)

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jkotas <6668460+jkotas@users.noreply.github.com>

Author: 3 people

src/libraries/Common/src/Interop/Windows/Interop.Errors.cs

@@ -26,6 +26,7 @@ internal static partial class Errors
         internal const int ERROR_HANDLE_EOF = 0x26;
         internal const int ERROR_NOT_SUPPORTED = 0x32;
         internal const int ERROR_BAD_NETPATH = 0x35;
+        internal const int ERROR_NETNAME_DELETED = 0x40;
         internal const int ERROR_NETWORK_ACCESS_DENIED = 0x41;
         internal const int ERROR_BAD_NET_NAME = 0x43;
         internal const int ERROR_FILE_EXISTS = 0x50;

src/libraries/Common/src/System/IO/FileSystem.Attributes.Windows.cs

@@ -121,6 +121,7 @@ Interop.Errors.ERROR_INVALID_NAME or
                 Interop.Errors.ERROR_BAD_PATHNAME or
                 Interop.Errors.ERROR_BAD_NETPATH or
                 Interop.Errors.ERROR_BAD_NET_NAME or
+                Interop.Errors.ERROR_NETNAME_DELETED or
                 Interop.Errors.ERROR_INVALID_PARAMETER or
                 Interop.Errors.ERROR_NETWORK_UNREACHABLE or
                 Interop.Errors.ERROR_NETWORK_ACCESS_DENIED or