fix: normalize developer role to system for OpenAI-format providers (#1011)

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

open-sse/translator/helpers/openaiHelper.js

@@ -10,6 +10,9 @@ export function filterToOpenAIFormat(body) {
   if (!body.messages || !Array.isArray(body.messages)) return body;
   
   body.messages = body.messages.map(msg => {
+    // Normalize developer role to system (many providers don't support developer)
+    if (msg.role === "developer") msg = { ...msg, role: "system" };
+    
     // Keep tool messages as-is (OpenAI format)
     if (msg.role === "tool") return msg;