Update LlmController and AiChat component to improve message clarity

- Changed progress update message in LlmController from "Sending request to LLM..." to "Thinking..." for better user understanding.
- Updated filtered messages in AiChat component to reflect the new progress update message, ensuring consistency in user experience.

src/Managing.Api/Controllers/LlmController.cs

@@ -348,7 +348,7 @@ public class LlmController : BaseController
             await PublishProgressUpdate(streamId, new LlmProgressUpdate
             {
                 Type = "thinking",
-                Message = "Sending request to LLM...",
+                Message = "Thinking...",
                 Iteration = iteration,
                 MaxIterations = maxIterations
             });

src/Managing.WebApp/src/components/organism/AiChat.tsx

@@ -109,7 +109,7 @@ function AiChat({ onClose }: AiChatProps): JSX.Element {
       // Messages to filter out (internal/system messages not useful for users)
       // These only appear when developerMode is enabled
       const filteredMessages = [
-        'Sending request to LLM...',
+        'Thinking...',
         'Waiting briefly to respect rate limits...',
         'Initializing conversation and loading available tools...'
       ]