fix(devti): remove ending newline and +``` from diff content

- Update DiffLangSketchProvider to handle diff content ending with "\n+```"
- Improve sketch creation for diff files by removing unnecessary ending

Author: phodal

core/src/main/kotlin/cc/unitmesh/devti/sketch/ui/patch/DiffLangSketchProvider.kt

@@ -6,5 +6,13 @@ import com.intellij.openapi.project.Project
 
 class DiffLangSketchProvider : LanguageSketchProvider {
     override fun isSupported(lang: String): Boolean = lang == "diff" || lang == "patch"
-    override fun create(project: Project, content: String): ExtensionLangSketch = DiffLangSketch(project, content)
+    override fun create(project: Project, content: String): ExtensionLangSketch {
+        val contentWithoutEnding = if (content.endsWith("\n+```")) {
+            content.substring(0, content.length - 4)
+        } else {
+            content
+        }
+
+        return DiffLangSketch(project, contentWithoutEnding)
+    }
 }