The github provider has format:"openai" (baseUrl .../chat/completions) plus a
separate responsesBaseUrl (.../responses); a model only uses the Responses API
when it sets targetFormat:"openai-responses". GitHub Copilot's Responses API
does not serve Claude/Gemini models, so claude-opus-4.7, claude-opus-4-5-20251101,
gemini-3.1-pro-preview and gemini-3-flash-preview failed with [400]. The working
claude-opus-4.6 carries no targetFormat and uses chat/completions.
Drop targetFormat:"openai-responses" from those four Claude/Gemini entries so
they use the provider default. Native OpenAI gpt-* models (and oswe-vscode-prime)
keep the Responses API.
Closes#2911
f14722315d