Fix panic when auto-importing relative .css augmentation from ESM file (#3845)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: DanielRosenwasser <972891+DanielRosenwasser@users.noreply.github.com>

Author: Copilot

internal/fourslash/tests/autoImportCssModule_test.go

@@ -0,0 +1,56 @@
+package fourslash_test
+
+import (
+	"testing"
+
+	"github.com/microsoft/typescript-go/internal/fourslash"
+	. "github.com/microsoft/typescript-go/internal/fourslash/tests/util"
+	"github.com/microsoft/typescript-go/internal/ls"
+	"github.com/microsoft/typescript-go/internal/lsp/lsproto"
+	"github.com/microsoft/typescript-go/internal/testutil"
+)
+
+func TestAutoImportCssModule(t *testing.T) {
+	t.Parallel()
+	defer testutil.RecoverAndFail(t, "Panic on fourslash test")
+	const content = `
+// @Filename: /tsconfig.json
+{ "compilerOptions": { "module": "nodenext", "moduleResolution": "nodenext" } }
+
+// @Filename: /package.json
+{ "type": "module" }
+
+// @Filename: /augmentations.ts
+export {};
+declare module "./styles.css" {
+    export const myClass: string;
+}
+
+// @Filename: /index.ts
+myClass/**/
+`
+	f, done := fourslash.NewFourslash(t, nil /*capabilities*/, content)
+	defer done()
+	// Verify auto-import completions don't panic when importing from .css module augmentation
+	f.VerifyCompletions(t, "", &fourslash.CompletionsExpectedList{
+		IsIncomplete: false,
+		ItemDefaults: &fourslash.CompletionsExpectedItemDefaults{
+			CommitCharacters: &DefaultCommitCharacters,
+			EditRange:        Ignored,
+		},
+		Items: &fourslash.CompletionsExpectedItems{
+			Includes: []fourslash.CompletionsExpectedItem{
+				&lsproto.CompletionItem{
+					Label: "myClass",
+					Data: &lsproto.CompletionItemData{
+						AutoImport: &lsproto.AutoImportFix{
+							ModuleSpecifier: "./styles.css",
+						},
+					},
+					AdditionalTextEdits: fourslash.AnyTextEdits,
+					SortText:            new(string(ls.SortTextAutoImportSuggestions)),
+				},
+			},
+		},
+	})
+}

internal/tspath/extension.go

@@ -1,7 +1,6 @@
 package tspath
 
 import (
-	"path/filepath"
 	"slices"
 	"strings"
 )
@@ -50,8 +49,8 @@ func RemoveFileExtension(path string) string {
 			return path[:len(path)-len(ext)]
 		}
 	}
-	// Otherwise just remove single dot extension, if any
-	return path[:len(path)-len(filepath.Ext(path))] //nolint:forbidigo
+
+	return path
 }
 
 func TryGetExtensionFromPath(p string) string {