renderers/qwen: tolerate non-leading system messages (#17757)

Coding clients may insert runtime system messages after the initial user turn. The shared Qwen renderer rejected these transcripts before rendering, turning a potentially usable non-standard request into an HTTP 500.

Pass non-leading system turns through the existing raw ChatML path and warn when qwen3.8 encounters one. Extend the Anthropic tool-route integration scenario to cover this message pattern and remove the obsolete rejection test.

integration/tools_routes_test.go

@@ -265,6 +265,7 @@ func runAnthropicToolRoute(t *testing.T, ctx context.Context, endpoint, model st
 		},
 		"messages": []any{
 			map[string]any{"role": "user", "content": []any{map[string]any{"type": "text", "text": "Call get_weather for Paris."}}},
+			map[string]any{"role": "system", "content": []any{map[string]any{"type": "text", "text": "Runtime token budget update."}}},
 			map[string]any{"role": "assistant", "content": []any{map[string]any{
 				"type":  "tool_use",
 				"id":    toolRoutePreviousID,

model/renderers/qwen35.go

@@ -193,16 +193,6 @@ func (r *Qwen35Renderer) validateMessages(messages []api.Message) error {
 		return fmt.Errorf("no user query found in messages")
 	}
 
-	for i, message := range messages {
-		switch message.Role {
-		case "system":
-			if i != 0 {
-				return fmt.Errorf("system message must be at the beginning")
-			}
-		case "user", "assistant", "tool":
-		}
-	}
-
 	return nil
 }
 
@@ -297,6 +287,9 @@ func (r *Qwen35Renderer) Render(messages []api.Message, tools []api.Tool, think
 		prefill := lastMessage && message.Role == "assistant"
 
 		if message.Role == "user" || (message.Role == "system" && i != 0) {
+			if r.variant == qwen35Renderer38 && message.Role == "system" {
+				slog.Warn("non-leading system message", "renderer", "qwen3.8")
+			}
 			sb.WriteString(imStartTag + message.Role + "\n" + content + imEndTag + "\n")
 		} else if message.Role == "assistant" {
 			renderAssistantThinkBlock := r.alwaysRenderAssistantThinkBlock || (isThinking && i > lastQueryIndex)

model/renderers/qwen38_test.go

@@ -364,14 +364,6 @@ func TestQwen38RendererRejectsInvalidTranscripts(t *testing.T) {
 			messages: []api.Message{{Role: "system", Content: "Hello"}},
 			wantErr:  "no user query found in messages",
 		},
-		{
-			name: "late system",
-			messages: []api.Message{
-				{Role: "user", Content: "Hello"},
-				{Role: "system", Content: "Late"},
-			},
-			wantErr: "system message must be at the beginning",
-		},
 		{
 			name: "system image",
 			messages: []api.Message{