fix the issue (microsoft#4388)

Author: SergeyMenshykh

dotnet/src/Microsoft.Agents.AI/Skills/FileAgentSkillsProvider.cs

@@ -174,7 +174,8 @@ private async Task<string> ReadSkillResourceAsync(string skillName, string resou
         {
             try
             {
-                promptTemplate = string.Format(optionsInstructions, string.Empty);
+                _ = string.Format(optionsInstructions, string.Empty);
+                promptTemplate = optionsInstructions;
             }
             catch (FormatException ex)
             {

dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/FileAgentSkillsProviderTests.cs

@@ -108,6 +108,8 @@ public async Task InvokingCoreAsync_CustomPromptTemplate_UsesCustomTemplateAsync
         // Assert
         Assert.NotNull(result.Instructions);
         Assert.StartsWith("Custom template:", result.Instructions);
+        Assert.Contains("custom-prompt-skill", result.Instructions);
+        Assert.Contains("Custom prompt", result.Instructions);
     }
 
     [Fact]