fix: remove tool call ID truncation and improve DeepSeek reasoning handling

The 40-character truncation of tool call IDs in helper.go caused collisions
when models (like deepseek-v4-flash) generated longer IDs, leading to
"Duplicate value for 'tool_call_id'" errors. Removed the limit to allow
full unique IDs.

DeepSeek: updated reasoning_content injection to use an empty string
instead of a space, better matching provider expectations for history.

Improved API error reporting across all providers by capturing raw body
content when response parsing fails or returns empty strings.

internal/providers/helpers.go

@@ -14,7 +14,12 @@ import (
 func MessagesToOpenAIJSON(messages []models.UnifiedMessage) ([]interface{}, error) {
 	var result []interface{}
 	for _, m := range messages {
-		if m.Role == "tool" {
+		role := strings.ToLower(m.Role)
+		if role == "model" {
+			role = "assistant"
+		}
+
+		if role == "tool" || role == "function" {
 			text := ""
 			if len(m.Content) > 0 {
 				text = m.Content[0].Text
@@ -23,13 +28,12 @@ func MessagesToOpenAIJSON(messages []models.UnifiedMessage) ([]interface{}, erro
 				"role":    "tool",
 				"content": text,
 			}
+			id := "unknown"
 			if m.ToolCallID != nil {
-				id := *m.ToolCallID
-				if len(id) > 40 {
-					id = id[:40]
-				}
-				msg["tool_call_id"] = id
+				id = *m.ToolCallID
 			}
+			msg["tool_call_id"] = id
+
 			if m.Name != nil {
 				msg["name"] = *m.Name
 			}
@@ -59,7 +63,9 @@ func MessagesToOpenAIJSON(messages []models.UnifiedMessage) ([]interface{}, erro
 		}
 
 		var finalContent interface{}
-		if len(parts) == 1 {
+		if len(parts) == 0 {
+			finalContent = nil
+		} else if len(parts) == 1 {
 			if p, ok := parts[0].(map[string]interface{}); ok && p["type"] == "text" {
 				finalContent = p["text"]
 			} else {
@@ -70,7 +76,7 @@ func MessagesToOpenAIJSON(messages []models.UnifiedMessage) ([]interface{}, erro
 		}
 
 		msg := map[string]interface{}{
-			"role":    m.Role,
+			"role":    role,
 			"content": finalContent,
 		}
 
@@ -82,20 +88,17 @@ func MessagesToOpenAIJSON(messages []models.UnifiedMessage) ([]interface{}, erro
 			sanitizedCalls := make([]models.ToolCall, len(m.ToolCalls))
 			copy(sanitizedCalls, m.ToolCalls)
 			for i := range sanitizedCalls {
-				if len(sanitizedCalls[i].ID) > 40 {
-					sanitizedCalls[i].ID = sanitizedCalls[i].ID[:40]
+				if sanitizedCalls[i].Type == "" {
+					sanitizedCalls[i].Type = "function"
 				}
 			}
 			msg["tool_calls"] = sanitizedCalls
-			if len(parts) == 0 {
-				msg["content"] = ""
-			}
+			msg["content"] = "" // OpenAI requirement: content must be string if tool_calls present
 		}
 
 		if m.Name != nil {
 			msg["name"] = *m.Name
 		}
-
 		result = append(result, msg)
 	}
 	return result, nil