Improve instrumentation test reliability with Compose waitUntil (#170)

Replace Thread.sleep calls with Compose's waitUntil for proper test synchronization, eliminating race conditions
Remove "Please Select a Model" popup dialog in favor of a system message in chat
Remove "Delete Chat History" confirmation dialog for streamlined UX
Add clearChatHistory() helper to ensure clean test state between tests
Fix various test timing issues by using proper Compose testing APIs

Author: kirklandsign