Use Go rules for Windows filename components

internal/engine/discovery_test.go

@@ -129,31 +129,33 @@ func TestFilenameComponentValidation(t *testing.T) {
 		}
 	}
 	if runtime.GOOS == "windows" {
-		for _, value := range []string{"CON", "nul.txt", "name.", "bad:name"} {
+		for _, value := range []string{"CON", "CONIN$", "CONOUT$", "name.", "bad:name", "bad\x01name"} {
 			if validFilenameComponent(value) {
 				t.Errorf("Windows component %q unexpectedly accepted", value)
 			}
 		}
-	} else {
-		for _, value := range []string{"name with spaces", "naïve"} {
-			if !validFilenameComponent(value) {
-				t.Errorf("host-valid component %q rejected", value)
+		// Device names with extensions depend on the Windows version. Match
+		// Go's platform-aware decision instead of imposing a blanket ban.
+		for _, value := range []string{"nul.txt", "COM¹.log", "LPT³.txt", "CON.extra.yml"} {
+			if got, want := validFilenameComponent(value), filepath.IsLocal(value); got != want {
+				t.Errorf("Windows component %q validity = %v, want Go's %v", value, got, want)
 			}
 		}
+	} else {
+		for _, value := range []string{"CONIN$", "CONOUT$", "nul.txt"} {
+			if !validFilenameComponent(value) {
+				t.Errorf("Windows component restriction applied to this host: %q", value)
+			}
+		}
+	}
+	for _, value := range []string{"name with spaces", "naïve"} {
+		if !validFilenameComponent(value) {
+			t.Errorf("host-valid component %q rejected", value)
+		}
 	}
 	if diagnostics := validateSearch(SearchDirectory("configs", DirectorySearchOptions{Filenames: []FilenameGroup{CustomFilenames("bad/name")}}), ""); len(diagnostics) == 0 {
 		t.Fatal("invalid custom filename was not retained for validation")
 	}
-	for _, value := range []string{"CON", "nul.txt", "COM¹.log", "LPT³.txt", "name.", "bad:name", "bad\x01name"} {
-		if validWindowsFilenameComponent(value) {
-			t.Errorf("Windows component %q unexpectedly accepted", value)
-		}
-	}
-	for _, value := range []string{"name with spaces", "naïve"} {
-		if !validWindowsFilenameComponent(value) {
-			t.Errorf("valid Windows component %q rejected", value)
-		}
-	}
 }
 
 func TestSearchCandidatesPreserveDirectoryAndGroupOrder(t *testing.T) {