Add POST /v1/messages, serving Anthropic SDK clients directly instead
of outbound-only support. A request resolved to an anthropic-type
provider passes through untranslated. A request resolved to any other
provider type translates to and from OpenAI's chat-completion shape
through the resolved adapter's own chatCompletion, since that method's
contract is OpenAI-shape in and out regardless of provider type.
The route reuses admitRequest, readAndDecodeUnifiedBody,
registry.resolve, withAttemptRecorder, and limiter.account exactly as
routes_unified.go does, so groups, model resolution, limits,
accounting, and caching semantics stay identical to the unified route.
Client-facing errors use Anthropic's envelope shape. A streaming
request is rejected explicitly with a 501, never silently answered
non-streamed; streaming support is a follow-up branch.
Auth reuses authStore.identify and logAuthEvent unchanged.
presentedKey already reads both x-api-key and Authorization: Bearer,
so no new credential-checking code was needed.
Add openAIRequestFromAnthropic and anthropicResponseFromOpenAI to
translate_anthropic.go, the mirror of the existing outbound
translators, plus their supporting helpers and unit tests.
