Custom models (added via the UI) on opencode-go / openai-compatible nodes always
routed as OpenAI-compatible because there was no per-model targetFormat: addCustomModel
didn't accept it, the API schema stripped it, and getModelTargetFormat (static-registry
only) never saw it — so a custom model needing the Anthropic Messages shape fell back to
the provider default 'openai'.
Thread an optional targetFormat through addCustomModel / replaceCustomModels /
updateCustomModel + the providerModelMutationSchema + the POST/PUT route, surface it from
getModelInfo (one combined custom-model metadata lookup alongside apiFormat), and use it
in chatCore's targetFormat resolution before the provider default.
Closes#2905
d4045e74b5