After #28660's host-gating fix, users with provider=custom and base_url
pointed at a commercial endpoint (DeepSeek, Groq, Mistral, …) hit
no-key-required even when they had the vendor-named env var set
(DEEPSEEK_API_KEY, GROQ_API_KEY, …). The issue author flagged this as
'what users intuitively expect'.
Adds _host_derived_api_key() to derive an env var name from the base URL
host using the *registrable* label (second-to-last). Appended to all three
api_key_candidates chains (_resolve_named_custom_runtime direct-alias path,
named-custom path, _resolve_openrouter_runtime non-openrouter branch).
Lookalike resistance: api.deepseek.com.attacker.test resolves to vendor
label 'attacker', NOT 'deepseek' — DEEPSEEK_API_KEY stays put. IPs and
loopback yield no vendor label. Already-handled vendors (OPENAI/OPENROUTER/
OLLAMA) are filtered to prevent bypass of the explicit host-gated paths.
Adds 6 tests covering positive paths (DeepSeek, Groq), the lookalike attack,
loopback rejection, the already-handled-vendor filter, and direct helper
unit tests.
Also adds erhnysr to AUTHOR_MAP.
c6a992e3e3