feat: Add Request.contentType() convenience accessor

Returns the Content-Type header as Optional<String> (blank treated as
absent, matching header(name)). The most frequently inspected header by
handler code; saves the magic-string typo risk.

Author: thced

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

@@ -102,6 +102,14 @@ public <T> T asPojo(Class<T> type) {
             + " does not support typed conversion; the mapper must implement TypedTypeMapper");
   }
 
+  /**
+   * Value of the {@code Content-Type} request header, or {@link Optional#empty()} if absent or
+   * blank. Convenience for {@code header("Content-Type")} — the most frequently inspected header.
+   */
+  public Optional<String> contentType() {
+    return header(CONTENT_TYPE);
+  }
+
   public String operationId() {
     return operationId;
   }

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

@@ -177,6 +177,28 @@ void queryParamBlankIsTreatedAsAbsent() {
     assertThat(req.queryParam("offset")).isEmpty();
   }
 
+  @Test
+  void contentTypeShortcutsContentTypeHeader() {
+    Request req =
+        new Request(
+            new byte[0],
+            null,
+            null,
+            "op",
+            Map.of(),
+            null,
+            headers("Content-Type", "application/json"));
+
+    assertThat(req.contentType()).contains("application/json");
+  }
+
+  @Test
+  void contentTypeEmptyWhenHeaderAbsent() {
+    Request req = new Request(new byte[0], null, null, "op", Map.of(), null, NO_HEADERS);
+
+    assertThat(req.contentType()).isEmpty();
+  }
+
   @Test
   void headerReturnsOptionalAndBlankIsAbsent() {
     Request req =