fix(opencode): only send stream_options with stream:true

OpenCode Go (and other strict OpenAI-compatible upstreams) reject a
non-streaming request that carries stream_options with
"stream_options should be set along with stream". The adapter attached it
unconditionally, so every non-stream call through the opencode adapter
failed on those upstreams.

Verified against OpenCode Go: 25/25 configured models now pass a real
completion through the gateway (they previously 400'd).

Test: TestOpenCodeStreamOptionsOnlyWhenStreaming (absent when
non-streaming, include_usage present when streaming).

internal/lua/toolcall_preservation_test.go

@@ -118,3 +118,36 @@ func TestAdaptersPreserveToolCalls(t *testing.T) {
 		}
 	}
 }
+
+// TestOpenCodeStreamOptionsOnlyWhenStreaming pins the fix for
+// "stream_options should be set along with stream": OpenCode Go (and other
+// strict OpenAI-compatible upstreams) reject a non-streaming request that
+// carries stream_options, which broke every non-stream call through the
+// opencode adapter.
+func TestOpenCodeStreamOptionsOnlyWhenStreaming(t *testing.T) {
+	vm := NewVM(freshAdapterDir(t))
+	if err := vm.Start(); err != nil {
+		t.Fatal(err)
+	}
+	defer vm.Stop()
+
+	// non-streaming: stream_options must be absent entirely
+	out, err := vm.Transform("opencode", "transform_request",
+		`{"model":"m","messages":[{"role":"user","content":"hi"}]}`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(out, "stream_options") {
+		t.Fatalf("non-streaming request must not carry stream_options: %s", out)
+	}
+
+	// streaming: stream_options.include_usage must be set
+	sout, err := vm.Transform("opencode", "transform_request",
+		`{"model":"m","stream":true,"messages":[{"role":"user","content":"hi"}]}`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(sout, "stream_options") || !strings.Contains(sout, "include_usage") {
+		t.Fatalf("streaming request must carry stream_options.include_usage: %s", sout)
+	}
+}