Select the appropriate dependency when Bazel's `os:windows` constraint is set.

The target `:default_event_engine_factory` currently uses gRPC specific config_settings which bundle the CPU with the OS (e.g. `cpu: windows_x86_64`). Use Bazel's OS constraint as one of the select cases so the correct target is used when settings `os: windows` as a constraint.

PiperOrigin-RevId: 693890511

BUILD

@@ -106,6 +106,11 @@ config_setting(
     values = {"apple_platform_type": "watchos"},
 )
 
+config_setting(
+    name = "windows_os",
+    constraint_values = ["@platforms//os:windows"],
+)
+
 config_setting(
     name = "systemd",
     values = {"define": "use_systemd=true"},

src/core/BUILD

@@ -2796,6 +2796,7 @@ grpc_cc_library(
     select_deps = [
         {
             "//:windows": ["windows_event_engine"],
+            "//:windows_os": ["windows_event_engine"],
             "//:windows_msvc": ["windows_event_engine"],
             "//:windows_clang": ["windows_event_engine"],
             "//:windows_other": ["windows_event_engine"],