feat: Add Builder.jsonMapper(TypeMapper) shortcut

bodyMapper("application/json", mapper) is the 95% case; jsonMapper(mapper)
removes the repeated media-type literal. Generic bodyMapper(mediaType, mapper)
stays for text/csv, application/xml, etc.

Author: thced

README.md

@@ -25,7 +25,7 @@ It is designed to be simple to use while providing the essential features needed
 - For `application/json` request/response bodies, either:
   - Gson on the classpath — auto-registered via the built-in `GsonJsonMapper` (integer-preserving, JSR-310 written as ISO-8601), or
   - Jackson via the built-in `JacksonJsonTypeMapper(ObjectMapper)` adapter (caller supplies a configured `ObjectMapper`), or
-  - any other `TypeMapper` you register via `Builder.bodyMapper("application/json", mapper)`.
+  - any other `TypeMapper` you register via `Builder.jsonMapper(mapper)` (shortcut for `bodyMapper("application/json", mapper)`).
 - Built-in mappers for `application/x-www-form-urlencoded` and `text/plain` need no configuration. Any other media type (`application/xml`, `application/cbor`, etc.) requires registering its own `TypeMapper`.
 
 
@@ -131,7 +131,7 @@ ObjectMapper objectMapper = new ObjectMapper()
 
 var server = OpenApiServer.builder()
     .spec(spec)
-    .bodyMapper("application/json", new JacksonJsonTypeMapper(objectMapper))
+    .jsonMapper(new JacksonJsonTypeMapper(objectMapper))
     .handlers(handlers)
     .build();
 ```

src/main/java/com/retailsvc/http/OpenApiServer.java

@@ -145,6 +145,10 @@ public Builder bodyMapper(String mediaType, TypeMapper mapper) {
       return this;
     }
 
+    public Builder jsonMapper(TypeMapper mapper) {
+      return bodyMapper("application/json", mapper);
+    }
+
     public Builder handlers(Map<String, RequestHandler> handlers) {
       this.handlers = handlers;
       return this;

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

@@ -88,6 +88,48 @@ public byte[] writeTo(Object v) {
     assertThat(readFromCalled).isTrue();
   }
 
+  @Test
+  void jsonMapperShortcutRegistersUnderApplicationJson() throws Exception {
+    AtomicBoolean readFromCalled = new AtomicBoolean();
+    TypeMapper marker =
+        new TypeMapper() {
+          @Override
+          public Object readFrom(byte[] b, String h) {
+            readFromCalled.set(true);
+            return Map.of("aList", List.of("x"), "feelingGood", true);
+          }
+
+          @Override
+          public byte[] writeTo(Object v) {
+            return new byte[0];
+          }
+        };
+    RequestHandler echo = req -> Response.status(200);
+    server =
+        OpenApiServer.builder()
+            .spec(spec)
+            .jsonMapper(marker)
+            .handlers(Map.of("get-data", echo, "post-data", echo))
+            .port(0)
+            .build();
+    HttpClient.newHttpClient()
+        .send(
+            HttpRequest.newBuilder()
+                .uri(URI.create("http://localhost:%d/api/v1/data".formatted(server.listenPort())))
+                .header("Content-Type", "application/json")
+                .POST(BodyPublishers.ofString("{\"anything\":\"goes\"}"))
+                .build(),
+            ofString());
+
+    assertThat(readFromCalled).isTrue();
+  }
+
+  @Test
+  void jsonMapperRejectsNullMapper() {
+    OpenApiServer.Builder b = OpenApiServer.builder();
+    assertThatThrownBy(() -> b.jsonMapper(null)).isInstanceOf(NullPointerException.class);
+  }
+
   @Test
   void bodyMapperRejectsNullArgs() {
     OpenApiServer.Builder b = OpenApiServer.builder();