Reject legacy empty function-call terminals

Why: The parser also recognizes function_call as a tool finish, but cannot execute a legacy call when no deltas were collected. It must fail closed like tool_calls.

Evidence: The function_call variant failed before this guard and passed afterward; full Agent-Native vet/test passed.

Impact: Inconsistent legacy tool terminals fail with returned Usage preserved; valid streams are unchanged.

Refs: U3 r0.4 independent review Important 2 closure

runtime/llm/sse.go

@@ -195,7 +195,7 @@ func emitParsed(ctx context.Context, r io.Reader, out chan<- Event) {
 	// A finish reason alone does not prove the tool arguments arrived intact.
 	// Preserve returned usage when rejecting a partial terminal, so the host
 	// can charge this call and stop rather than treating it as a free retry.
-	if finished == "tool_calls" && len(order) == 0 {
+	if (finished == "tool_calls" || finished == "function_call") && len(order) == 0 {
 		sendTerminalFailure("tool call finish reason without a tool call")
 		return
 	}

runtime/llm/sse_test.go

@@ -94,8 +94,10 @@ func TestParseSSEToolCallIncrements(t *testing.T) {
 }
 
 func TestParseSSEToolFinishWithoutCallFails(t *testing.T) {
+	for _, reason := range []string{"tool_calls", "function_call"} {
+		t.Run(reason, func(t *testing.T) {
 			body := strings.Join([]string{
-		`data: {"choices":[{"delta":{},"finish_reason":"tool_calls"}],"usage":{"prompt_tokens":12,"completion_tokens":5,"total_tokens":17}}`,
+				`data: {"choices":[{"delta":{},"finish_reason":"` + reason + `"}],"usage":{"prompt_tokens":12,"completion_tokens":5,"total_tokens":17}}`,
 				`data: [DONE]`,
 				"",
 			}, "\n")
@@ -109,6 +111,8 @@ func TestParseSSEToolFinishWithoutCallFails(t *testing.T) {
 				!end.StreamComplete || len(end.Message.ToolCalls) != 0 {
 				t.Fatalf("empty tool finish must fail and retain Usage: %#v", end)
 			}
+		})
+	}
 }
 
 func TestParseSSELengthFailsUnfinishedToolCall(t *testing.T) {