Update load testing with guest authentication and all business flows

[locnguyen1986]

Authentication & Environment

• Removed API key dependencies - All tests now use guest authentication automatically
• Updated default URL from api-dev.jan.ai to api-stag.jan.ai (staging environment)
• Eliminated secrets requirements - No more LOADTEST_API_KEY or LOADTEST_TOKEN needed

Test Structure Modernization: Replaced monolithic test with three focused test scripts:

• test-completion-standard.js - Basic completion flows
• test-completion-conversation.js - Conversation management with streaming/non-streaming
• test-responses.js - Response API endpoint testing

[locnguyen1986]

The grafana dashboard: https://grafana.menlo.ai/d/lkkajsdfq234124asd/jan-server-load-test-k6-and-llm-metrics