fix(tau): route user simulator through DigitalOcean inference

[pvennacherla]

Summary

• fix the TAU Airline user simulator to use openai-gpt-5.4-mini through DigitalOcean inference
• reuse the candidate token for DigitalOcean production/test endpoints and require a separate launch-only simulator token for OpenRouter/custom endpoints
• update dashboard validation, diagnostic retries, tests, and API documentation

Validation

• bun run format:check
• bun run check
• bun run typecheck
• GIT_CONFIG_GLOBAL=/dev/null bun test (1,613 passed)
• bun run build