fix: Variable is not supported in a snippet placeholder

Fixes #1313

Signed-off-by: azerr <azerr@redhat.com>

Author: angelozerr

src/main/java/com/redhat/devtools/lsp4ij/client/features/LSPCompletionProposal.java

@@ -435,6 +435,10 @@ public CompletionItem getItem() {
      * @return the response of the resolved LSP variable and null otherwise.
      */
     private @Nullable String getVariableValue(String variableName) {
+        if (!variableName.isEmpty() && variableName.charAt(0) == '$') {
+            // Remove $
+            variableName = variableName.substring(1);
+        }
         Document document = editor.getDocument();
         switch (variableName) {
             case TM_FILENAME_BASE:

src/test/java/com/redhat/devtools/lsp4ij/features/completion/VariableCompletionTest.java

@@ -0,0 +1,110 @@
+/*******************************************************************************
+ * Copyright (c) 2025 Red Hat, Inc.
+ * Distributed under license by Red Hat, Inc. All rights reserved.
+ * This program is made available under the terms of the
+ * Eclipse Public License v2.0 which accompanies this distribution,
+ * and is available at http://www.eclipse.org/legal/epl-v20.html
+ *
+ * Contributors:
+ * Red Hat, Inc. - initial API and implementation
+ ******************************************************************************/
+package com.redhat.devtools.lsp4ij.features.completion;
+
+import com.redhat.devtools.lsp4ij.fixtures.LSPCompletionFixtureTestCase;
+
+/**
+ * Completion tests with 'textDocument/completion' responses
+ * which returns snippets with LSP standard variables}.
+ *
+ * @see {@link com.redhat.devtools.lsp4ij.features.completion.snippet.LspSnippetVariableConstants}
+ */
+public class VariableCompletionTest extends LSPCompletionFixtureTestCase {
+
+    public VariableCompletionTest() {
+        super("*.ts");
+    }
+
+    public void testCompletionWith$TM_FILENAME() {
+        // 1. Test completion items response
+        assertCompletion("MyComponent.ts",
+                "<caret>", // language=json
+                """                
+                
+                        {
+                        "isIncomplete": false,
+                        "items": [
+                          {
+                            "label": "Type",
+                            "kind": 15,
+                            "insertTextFormat": 2,
+                            "insertTextMode": 2,
+                            "textEdit": {
+                              "range": {
+                                "start": {
+                                  "line": 1,
+                                  "character": 1
+                                },
+                                "end": {
+                                  "line": 1,
+                                  "character": 1
+                                }
+                              },
+                              "newText": "Type ${1:$TM_FILENAME} {\\n    |\\n}"
+                            }
+                          }
+                        ]
+                      }"""
+                ,
+                "Type");
+        // 2. Test new editor content after applying the first completion item
+
+        // Apply completion with Type
+        assertApplyCompletionItem(0,
+                """
+                        Type MyComponent.ts {
+                            |
+                        }""");
+    }
+
+    public void testCompletionWith$TM_FILENAME_BASE() {
+        // 1. Test completion items response
+        assertCompletion("MyComponent.ts",
+                        "<caret>", // language=json
+                        """                
+                        
+                                {
+                                "isIncomplete": false,
+                                "items": [
+                                  {
+                                    "label": "Type",
+                                    "kind": 15,
+                                    "insertTextFormat": 2,
+                                    "insertTextMode": 2,
+                                    "textEdit": {
+                                      "range": {
+                                        "start": {
+                                          "line": 1,
+                                          "character": 1
+                                        },
+                                        "end": {
+                                          "line": 1,
+                                          "character": 1
+                                        }
+                                      },
+                                      "newText": "Type ${1:$TM_FILENAME_BASE} {\\n    |\\n}"
+                                    }
+                                  }
+                                ]
+                              }"""
+                ,
+                "Type");
+        // 2. Test new editor content after applying the first completion item
+
+        // Apply completion with Type
+        assertApplyCompletionItem(0,
+                """
+                        Type MyComponent {
+                            |
+                        }""");
+    }
+}