test(01-02): add failing test for optional plugin services

- Cover typed Publish/Lookup, duplicate providers and HasPlugin
- Require missing/cycle errors to name plugin IDs before Boot
- Assert greeter works with and without an optional extra

examples/hello/plugins/greeter/plugin_test.go

@@ -0,0 +1,75 @@
+package greeter
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"io"
+	"strings"
+	"testing"
+
+	"git.golem15.com/golem15/summercms/backpack"
+	"git.golem15.com/golem15/summercms/bonfire"
+	"git.golem15.com/golem15/summercms/compass"
+	"git.golem15.com/golem15/summercms/pact"
+)
+
+type testOut struct{ bytes.Buffer }
+
+func (o *testOut) Write(p []byte) (int, error) { return o.Buffer.Write(p) }
+func (o *testOut) Println(a ...any)            { fmt.Fprintln(&o.Buffer, a...) }
+func (o *testOut) Printf(format string, a ...any) {
+	fmt.Fprintf(&o.Buffer, format, a...)
+}
+
+var _ bonfire.Output = (*testOut)(nil)
+var _ io.Writer = (*testOut)(nil)
+
+type extraMsg struct{ msg string }
+
+func (e extraMsg) Message() string { return e.msg }
+
+func TestBootSkipsAbsentOptionalPlugin(t *testing.T) {
+	app := backpack.New(&compass.Config{})
+	app.SetPlugins([]string{"golem15.hello", "golem15.greeter"})
+	p := &Plugin{}
+	if err := p.Boot(app); err != nil {
+		t.Fatalf("Boot: %v", err)
+	}
+	out := runHelloCmd(t, p)
+	if strings.Contains(out, "hello-from-optional") {
+		t.Fatalf("absent optional leaked into output: %q", out)
+	}
+	if !strings.Contains(out, "extra=") {
+		t.Fatalf("output %q should include extra=", out)
+	}
+}
+
+func TestBootUsesPublishedOptionalService(t *testing.T) {
+	app := backpack.New(&compass.Config{})
+	app.SetPlugins([]string{"golem15.hello", "golem15.greeter", "golem15.optional"})
+	if err := app.Publish[pact.OptionalMessage](extraMsg{msg: "hello-from-optional"}); err != nil {
+		t.Fatalf("Publish: %v", err)
+	}
+	p := &Plugin{}
+	if err := p.Boot(app); err != nil {
+		t.Fatalf("Boot: %v", err)
+	}
+	out := runHelloCmd(t, p)
+	if !strings.Contains(out, "extra=hello-from-optional") {
+		t.Fatalf("output = %q, want extra=hello-from-optional", out)
+	}
+}
+
+func runHelloCmd(t *testing.T, p *Plugin) string {
+	t.Helper()
+	cmds := p.Commands()
+	if len(cmds) == 0 {
+		t.Fatal("no commands")
+	}
+	var out testOut
+	if err := cmds[0].Run(context.Background(), nil, &out); err != nil {
+		t.Fatalf("Run: %v", err)
+	}
+	return strings.TrimSpace(out.String())
+}