Add hasContentType matcher, HTTP status consts

This matcher removes a lot of boilerplate from checking the Content-Type
of a HttpResponse. It's based on the Hamcrest API with the help of these
tutorials:

- https://hamcrest.org/JavaHamcrest/tutorial
- https://www.baeldung.com/hamcrest-custom-matchers

This could be generalized to handle any header.  However, I want to keep
it straightforward for now, as a good basic example of how to write a
custom matcher.

Also took the opportunity to replace bare HTTP status codes (200, et.
al.) with constants defined on jakarta.servlet.http.HttpServletResponse.

strcalc/src/test/java/com/mike_bland/training/testing/matchers/HasContentType.java

@@ -0,0 +1,40 @@
+package com.mike_bland.training.testing.matchers;
+
+import org.hamcrest.Description;
+import org.hamcrest.TypeSafeMatcher;
+
+import java.net.http.HttpResponse;
+
+// Custom Hamcrest matcher validating the Content-Type header of a HttpResponse.
+//
+// This could be generalized to check any HTTP header, or collection thereof.
+// However, since this is a teaching example, we'll keep it straightforward.
+class HasContentType<T> extends TypeSafeMatcher<HttpResponse<T>> {
+    private final String expected;
+
+    // Constructor to register the expected Content-Type value.
+    HasContentType(String contentType) {
+        this.expected = contentType;
+    }
+
+    // Helper method to extract the Content-Type value from an HttpResponse.
+    private String getContentType(HttpResponse<T> resp) {
+        return resp.headers().firstValue("Content-Type").orElse("");
+    }
+
+    // Performs the actual assertion.
+    @Override public boolean matchesSafely(HttpResponse<T> resp) {
+        return getContentType(resp).equals(expected);
+    }
+
+    // Describes the "Expected:" value in assertion failure messages.
+    @Override public void describeTo(Description description) {
+        description.appendText(expected);
+    }
+
+    // Describes the actual ("but:") value in assertion failure messages.
+    @Override public void describeMismatchSafely(
+            HttpResponse<T> resp, Description description) {
+        description.appendText(getContentType(resp));
+    }
+}

strcalc/src/test/java/com/mike_bland/training/testing/matchers/Matchers.java

@@ -0,0 +1,23 @@
+package com.mike_bland.training.testing.matchers;
+
+import org.hamcrest.Matcher;
+
+import java.net.http.HttpResponse;
+
+// Collection of custom Hamcrest Matcher<T> classes for assertThat() statements.
+//
+// These tutorials describe how to write Matchers:
+//
+// - https://hamcrest.org/JavaHamcrest/tutorial
+// - https://www.baeldung.com/hamcrest-custom-matchers
+//
+// Note that these tutorials show the static factory functions defined on the
+// same class as the Matcher. However, Hamcrest itself collects these factories
+// into its own org.hamcrest.Matchers class for convenience, instead of
+// importing one class per Matcher.
+public class Matchers {
+    public static <T> Matcher<HttpResponse<T>> hasContentType(
+            String contentType) {
+        return new HasContentType<>(contentType);
+    }
+}

strcalc/src/test/java/com/mike_bland/training/testing/stringcalculator/ServletContractTest.java

@@ -10,6 +10,7 @@
 import com.mike_bland.training.testing.annotations.MediumTest;
 import com.mike_bland.training.testing.utils.PortPicker;
 import com.mike_bland.training.testing.utils.TestTomcat;
+import jakarta.servlet.http.HttpServletResponse;
 import org.junit.jupiter.api.AfterAll;
 import org.junit.jupiter.api.BeforeAll;
 
@@ -20,6 +21,7 @@
 import java.net.http.HttpResponse.BodyHandlers;
 import java.util.Optional;
 
+import static com.mike_bland.training.testing.matchers.Matchers.hasContentType;
 import static com.mike_bland.training.testing.stringcalculator.Servlet.DEFAULT_ROOT;
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.containsString;
@@ -65,11 +67,8 @@ void servesCorrectLandingPage() throws Exception {
 
         var resp = sendRequest(req);
 
-        assertEquals(200, resp.statusCode());
-        assertEquals(
-                Optional.of("text/html"),
-                resp.headers().firstValue("Content-Type")
-        );
+        assertEquals(HttpServletResponse.SC_OK, resp.statusCode());
+        assertThat(resp, hasContentType("text/html"));
         assertThat(
                 resp.body(),
                 containsString("<title>String Calculator - ")
@@ -86,11 +85,8 @@ void addEndpointPlaceholder() throws Exception {
 
         var resp = sendRequest(req);
 
-        assertEquals(200, resp.statusCode());
-        assertEquals(
-                Optional.of("text/plain;charset=UTF-8"),
-                resp.headers().firstValue("Content-Type")
-        );
+        assertEquals(HttpServletResponse.SC_OK, resp.statusCode());
+        assertThat(resp, hasContentType("text/plain;charset=UTF-8"));
         assertEquals("placeholder for /add API endpoint", resp.body());
     }
 }