refactor: Rename ContentTypeHeader.subtype to mediaType

The method returns the full media type (e.g. "application/json"),
not the RFC 9110 subtype (e.g. "json"). Rename the method, its
single caller's local variable, and the test method names to match
what's actually returned.

Author: thced

src/main/java/com/retailsvc/http/internal/ContentTypeHeader.java

@@ -12,7 +12,7 @@ private ContentTypeHeader() {}
    * Returns the bare media type, stripping parameters and lower-casing for case-insensitive
    * matching (RFC 9110 / 2045). {@code null} → {@code application/json}.
    */
-  public static String subtype(String header) {
+  public static String mediaType(String header) {
     if (header == null) {
       return "application/json";
     }

src/main/java/com/retailsvc/http/internal/RequestPreparationFilter.java

@@ -143,15 +143,15 @@ private Object validateAndParseBody(HttpExchange exchange, Operation op, byte[]
       return null;
     }
     String header = exchange.getRequestHeaders().getFirst("Content-Type");
-    String subtype = ContentTypeHeader.subtype(header);
-    MediaType mt = rb.get().content().get(subtype);
+    String mediaType = ContentTypeHeader.mediaType(header);
+    MediaType mt = rb.get().content().get(mediaType);
     if (mt == null) {
       throw new ValidationException(
           new ValidationError(
-              "/body", "content-type", "unsupported content type: " + subtype, null));
+              "/body", "content-type", "unsupported content type: " + mediaType, null));
     }
     Object parsed =
-        switch (subtype) {
+        switch (mediaType) {
           case "application/x-www-form-urlencoded" ->
               formParser.parseAndCoerce(body, header, mt.schema());
           case "text/plain" -> textParser.parse(body, header);

src/test/java/com/retailsvc/http/internal/ContentTypeHeaderTest.java

@@ -7,29 +7,29 @@
 class ContentTypeHeaderTest {
 
   @Test
-  void subtypeReturnsBareMediaType() {
-    assertThat(ContentTypeHeader.subtype("application/json")).isEqualTo("application/json");
+  void mediaTypeReturnsBareValue() {
+    assertThat(ContentTypeHeader.mediaType("application/json")).isEqualTo("application/json");
   }
 
   @Test
-  void subtypeStripsParameters() {
-    assertThat(ContentTypeHeader.subtype("text/plain; charset=utf-8")).isEqualTo("text/plain");
+  void mediaTypeStripsParameters() {
+    assertThat(ContentTypeHeader.mediaType("text/plain; charset=utf-8")).isEqualTo("text/plain");
   }
 
   @Test
-  void subtypeTrimsWhitespace() {
-    assertThat(ContentTypeHeader.subtype("  application/json  ")).isEqualTo("application/json");
+  void mediaTypeTrimsWhitespace() {
+    assertThat(ContentTypeHeader.mediaType("  application/json  ")).isEqualTo("application/json");
   }
 
   @Test
-  void subtypeDefaultsToApplicationJsonWhenNull() {
-    assertThat(ContentTypeHeader.subtype(null)).isEqualTo("application/json");
+  void mediaTypeDefaultsToApplicationJsonWhenNull() {
+    assertThat(ContentTypeHeader.mediaType(null)).isEqualTo("application/json");
   }
 
   @Test
-  void subtypeLowerCasesMediaType() {
-    assertThat(ContentTypeHeader.subtype("Application/JSON")).isEqualTo("application/json");
-    assertThat(ContentTypeHeader.subtype("Text/Plain; charset=UTF-8")).isEqualTo("text/plain");
+  void mediaTypeLowerCasesValue() {
+    assertThat(ContentTypeHeader.mediaType("Application/JSON")).isEqualTo("application/json");
+    assertThat(ContentTypeHeader.mediaType("Text/Plain; charset=UTF-8")).isEqualTo("text/plain");
   }
 
   @Test