feat(01-02): add optional plugin services and HasPlugin

- App-scoped typed Publish/Lookup with duplicate-provider errors
- Set plugin IDs before Register so HasPlugin sees the full set
- Greeter uses pact.OptionalMessage without importing optional

party/registry_test.go

@@ -126,11 +126,11 @@ func TestActivateDuplicateIDRejected(t *testing.T) {
 }
 
 type bootProbe struct {
-	id      string
-	req     []string
-	log     *[]string
-	sawOpt  *bool
-	booted  *[]string
+	id     string
+	req    []string
+	log    *[]string
+	sawOpt *bool
+	booted *[]string
 }
 
 func (p *bootProbe) ID() string         { return p.id }