feat: implement AI gateway phase one runtime

apps/api/internal/httpapi/streaming.go

@@ -0,0 +1,45 @@
+package httpapi
+
+import "net/http"
+
+func writeCompatibleStream(w http.ResponseWriter, kind string, model string, output map[string]any) {
+	w.Header().Set("Content-Type", "text/event-stream")
+	w.Header().Set("Cache-Control", "no-cache")
+	w.Header().Set("Connection", "keep-alive")
+	content := extractOutputText(output)
+	if content == "" {
+		content = "done"
+	}
+	if kind == "responses" {
+		sendSSE(w, "response.output_text.delta", map[string]any{"type": "response.output_text.delta", "delta": content})
+		sendSSE(w, "response.completed", map[string]any{"type": "response.completed", "response": output})
+		return
+	}
+	sendSSE(w, "message", map[string]any{
+		"id":      output["id"],
+		"object":  "chat.completion.chunk",
+		"model":   model,
+		"choices": []any{map[string]any{"index": 0, "delta": map[string]any{"content": content}, "finish_reason": nil}},
+	})
+	sendSSE(w, "message", map[string]any{
+		"id":      output["id"],
+		"object":  "chat.completion.chunk",
+		"model":   model,
+		"choices": []any{map[string]any{"index": 0, "delta": map[string]any{}, "finish_reason": "stop"}},
+	})
+}
+
+func extractOutputText(output map[string]any) string {
+	if text, ok := output["output_text"].(string); ok {
+		return text
+	}
+	choices, _ := output["choices"].([]any)
+	for _, rawChoice := range choices {
+		choice, _ := rawChoice.(map[string]any)
+		message, _ := choice["message"].(map[string]any)
+		if content, ok := message["content"].(string); ok {
+			return content
+		}
+	}
+	return ""
+}