Claude models on Vertex AI were being sent through the generic OpenAI-
compatible partner endpoint, which 404s/errors for Claude on at least
some projects. Route them through Vertex's native Anthropic Messages
API (publishers/anthropic/.../rawPredict) instead, stripping the
body-level model field rawPredict rejects and injecting the required
anthropic_version field.
rawPredict only ever returns a complete JSON body, never real SSE
framing, so streaming requests now get a genuine Anthropic-format SSE
stream synthesized from that JSON (message_start/content_block_*/
message_delta/message_stop), which the existing claude-to-openai
response translator already knows how to parse.
Also fixes two response-format resolution bugs that silently dropped
a custom model's DB-stored targetFormat override whenever the model
id also existed in the static provider registry (as claude-sonnet-4-6
and claude-opus-4-7 do under vertex): resolveModelOrError had its own
ad-hoc resolution that never consulted the override, and even once
fixed, executeChatWithBreaker discarded the correctly-resolved format
before handleChatCore's own resolution ran a second time.
1e629721b9