feat(vscode-ide-companion/auth): deduplicate concurrent authentication calls

Prevent multiple simultaneous authentication flows by:
- Adding static authInFlight promise tracking in AcpConnection
- Implementing runExclusiveAuth method in AuthStateManager
- Adding sessionCreateInFlight tracking in QwenAgentManager
- Ensuring only one auth flow runs at a time across different components

This prevents race conditions and duplicate login prompts when multiple components request authentication simultaneously.

packages/vscode-ide-companion/src/webview/styles/styles.css

@@ -5,7 +5,6 @@
  */
 
 /* Import component styles */
-@import '../components/messages/Assistant/AssistantMessage.css';
 @import './timeline.css';
 @import '../components/messages/MarkdownRenderer/MarkdownRenderer.css';