fix: use non-whitespace placeholder in chatprompttemplate constructor

The base class PromptTemplateBase validates that the template is not empty
or whitespace. Changed from " " (space) to "{placeholder}" to pass validation.

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Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: ooples

src/PromptEngineering/Templates/ChatPromptTemplate.cs

@@ -46,7 +46,7 @@ public class ChatPromptTemplate : PromptTemplateBase
     /// </summary>
     /// <param name="messageFormat">Format for each message (default: "{role}: {content}").</param>
     public ChatPromptTemplate(string? messageFormat = null)
-        : base(" ") // Base class requires a non-empty template; we rebuild it immediately.
+        : base("{placeholder}") // Base class requires a non-whitespace template; we rebuild it immediately.
     {
         _messages = new List<ChatMessage>();
         _messageFormat = messageFormat ?? "{role}: {content}";