refactor(test): relocate serialization golden data to tests/serialization

Move the serialization golden fixtures out of server/types/testdata/golden into
tests/serialization (alongside the other test fixtures under tests/), and rename
server/types/serialization_golden_test.go to serialization_test.go. The test stays
in package types so it keeps running under `go test ./server/...` (make test).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: gwleclerc

server/types/serialization_golden_test.go server/types/serialization_test.goserver/types/serialization_golden_test.go renamed to server/types/serialization_test.go

@@ -39,7 +39,7 @@ var updateGolden = flag.Bool("update-golden", false, "regenerate golden serializ
 const (
 	dataDir     = "../../tests/data"
 	sessionsDir = "../../tests/sessions"
-	goldenDir   = "testdata/golden"
+	goldenDir   = "../../tests/serialization"
 )
 
 // loadFn reads a fixture file and returns a freshly-populated value of the right type.