Refs #8932: add Claude design provider fallback

[T-rav]

Summary

• add an httpx-based Anthropic Messages provider for onboarding design chat when HYDRAFLOW_ONBOARDING_ANTHROPIC_API_KEY or ANTHROPIC_API_KEY is configured
• keep deterministic field extraction as no-key and live-provider failure fallback
• retry malformed Claude JSON once, sanitize field updates to BootstrapSpec fields, and record fallback events on drafts
• update direct and route tests for async design chat/provider behavior

Verification

• uv run pytest tests/test_onboarding_api.py tests/test_onboarding_design_ai.py -q
• uv run ruff check src/onboarding/design_ai.py src/dashboard_routes/_onboarding_routes.py tests/test_onboarding_api.py tests/test_onboarding_design_ai.py
• uv run pyright src/onboarding/design_ai.py src/dashboard_routes/_onboarding_routes.py
• make quality

Refs #8932