refactor: Reuse EchoHandler for Shapes integration tests

Delete PolymorphicHandler, which had no unique behaviour over EchoHandler
(both return 200 and reflect the request body). Update all four handler
references in the Shapes nested class to use EchoHandler directly. Adjust
the circle body assertion to check the echoed payload ("kind":"circle")
instead of the old hard-coded {"ok":true} response. Strengthen the
missing-discriminator test with Content-Type and body assertions to mirror
the unknown-kind case.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: thced

src/test/java/com/retailsvc/http/OpenApiServerIT.java

@@ -410,16 +410,15 @@ class Shapes {
 
     @Test
     void postShape_validCircleReturns200() {
-      try (var server =
-              newServer(Map.of("post-shape", new com.retailsvc.http.start.PolymorphicHandler()));
+      try (var server = newServer(Map.of("post-shape", new EchoHandler()));
           var client = httpClient()) {
         var body = "{\"kind\":\"circle\",\"radius\":2.5}";
         var request = newRequest(server, path, "POST", ofString(body));
 
         var response = client.send(request, BodyHandlers.ofString());
 
         assertThat(response.statusCode()).isEqualTo(200);
-        assertThat(response.body()).contains("\"ok\":true");
+        assertThat(response.body()).contains("\"kind\":\"circle\"");
       } catch (IOException e) {
         fail(e);
       } catch (InterruptedException e) {
@@ -430,8 +429,7 @@ void postShape_validCircleReturns200() {
 
     @Test
     void postShape_validSquareReturns200() {
-      try (var server =
-              newServer(Map.of("post-shape", new com.retailsvc.http.start.PolymorphicHandler()));
+      try (var server = newServer(Map.of("post-shape", new EchoHandler()));
           var client = httpClient()) {
         var body = "{\"kind\":\"square\",\"side\":3}";
         var request = newRequest(server, path, "POST", ofString(body));
@@ -450,8 +448,7 @@ void postShape_validSquareReturns200() {
     @Test
     void postShape_unknownKindReturns400() {
       // matches zero branches: "kind" is neither "circle" nor "square".
-      try (var server =
-              newServer(Map.of("post-shape", new com.retailsvc.http.start.PolymorphicHandler()));
+      try (var server = newServer(Map.of("post-shape", new EchoHandler()));
           var client = httpClient()) {
         var body = "{\"kind\":\"triangle\",\"side\":3}";
         var request = newRequest(server, path, "POST", ofString(body));
@@ -473,15 +470,17 @@ void postShape_unknownKindReturns400() {
     @Test
     void postShape_missingDiscriminatorReturns400() {
       // omitting "kind" makes both branches fail "required".
-      try (var server =
-              newServer(Map.of("post-shape", new com.retailsvc.http.start.PolymorphicHandler()));
+      try (var server = newServer(Map.of("post-shape", new EchoHandler()));
           var client = httpClient()) {
         var body = "{\"radius\":2.5}";
         var request = newRequest(server, path, "POST", ofString(body));
 
         var response = client.send(request, BodyHandlers.ofString());
 
         assertThat(response.statusCode()).isEqualTo(400);
+        assertThat(response.headers().firstValue("Content-Type").orElse(""))
+            .contains("application/problem+json");
+        assertThat(response.body()).contains("oneOf");
       } catch (IOException e) {
         fail(e);
       } catch (InterruptedException e) {