Two gaps for custom providers behind a WAF/CDN:
- `build_anthropic_client` never consulted `custom_providers[].extra_headers`,
so a relay in `anthropic_messages` mode that rejects the SDK User-Agent kept
403ing even with `extra_headers: {User-Agent: ...}` configured, while the
OpenAI-wire clients already applied it. The lookup now lives in
`_new_sdk_client`, the one constructor every builder path goes through
(init, /model switch, rebuild, auxiliary), keyed by the caller's raw route
because entries are keyed by the `/v1` form the normalizer strips.
Salvaged direction of #46002 (@wait4xx). Fixes#24293, #9721.
- `_status_403` classified every non-billing 403 as `auth`, so a WAF's plain
"Your request was blocked." or a Cloudflare browser challenge printed "Your
API key was rejected" and could rotate a healthy credential. A 403 carrying
established block/challenge markers is now `upstream_blocked`: no rotation,
no retry, fallback allowed, WAF/User-Agent guidance on every surface (CLI
loop, chat copy, cli chat error copy, TUI gateway + Ink TUI copy). Generic
403 and all 401 keep the auth verdict. Salvaged direction of #70567
(@ooiuuii) and #53114 (@AgenticSpark). Fixes#53099, #70566.
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