Translator: remove Copilot mention in OpenAI->Claude stream comment

The oauth hardening PR (#662) originally removed a Copilot mention in internal/translator/openai/claude/openai_claude_response.go, but the translator-path-guard blocks PRs from touching internal/translator. Please update this comment in a maintainer-owned change (suggested wording: "Some providers may send tool_calls in the first chunk without a role field.").