Deepseek API (and likely other providers) reject messages with
role: 'developer' — only accept system, user, assistant, tool.
filterToOpenAIFormat() normalizes content blocks but never touched
message roles, so developer passed through unmodified and caused
400 errors (issue #773).
Fix: add one-line developer → system mapping in filterToOpenAIFormat()
before role-specific logic. This is the common normalization point
called for all targetFormat=openai providers (Deepseek, Groq, Mistral,
Perplexity, Together, Fireworks, Cerebras, xAI, NVIDIA, etc.)
Closes#773
