fix(byok): improve Gemini provider error handling and add tests#2686

dmitrivMS merged 3 commits intomicrosoft:mainmicrosoft/vscode-copilot-chat:mainfrom

bharatvansh:fix/gemini-byok-error-handlingbharatvansh/vscode-copilot-chat:fix/gemini-byok-error-handlingCopy head branch name to clipboard