Add Cookie attributes + SameSite CookieResultMatchers in MockMvc

This commit adds assertions to MockMvc's CookieresultMatchers:
 - `attribute` for arbitrary attributes
 - `sameSite` for the SameSite well-known attribute

Note that the `sameSite` methods delegate to their `attribute`
counterparts. Note also that Jakarta's `Cookie#getAttribute` method is
case-insensitive, which is reflected in the documentation of the
`attribute` assertion method and the tests.

Closes gh-30285

spring-test/src/main/java/org/springframework/test/web/servlet/result/CookieResultMatchers.java

@@ -146,6 +146,24 @@ public class CookieResultMatchers {
 		};
 	}
 
+	/**
+	 * Assert a cookie's SameSite attribute with a Hamcrest {@link Matcher}.
+	 * @since 6.0.8
+	 * @see #attribute(String, String, Matcher)
+	 */
+	public ResultMatcher sameSite(String name, Matcher<? super String> matcher) {
+		return attribute(name, "SameSite", matcher);
+	}
+
+	/**
+	 * Assert a cookie's SameSite attribute.
+	 * @since 6.0.8
+	 * @see #attribute(String, String, String)
+	 */
+	public ResultMatcher sameSite(String name, String sameSite) {
+		return attribute(name, "SameSite", sameSite);
+	}
+
 	/**
 	 * Assert a cookie's comment with a Hamcrest {@link Matcher}.
 	 */
@@ -211,6 +229,34 @@ public class CookieResultMatchers {
 		};
 	}
 
+	/**
+	 * Assert a cookie's specified attribute with a Hamcrest {@link Matcher}.
+	 * @param cookieAttribute the name of the Cookie attribute (case-insensitive)
+	 * @since 6.0.8
+	 */
+	public ResultMatcher attribute(String cookieName, String cookieAttribute, Matcher<? super String> matcher) {
+		return result -> {
+			Cookie cookie = getCookie(result, cookieName);
+			String attribute = cookie.getAttribute(cookieAttribute);
+			assertNotNull("Response cookie '" + cookieName + "' doesn't have attribute '" + cookieAttribute + "'", attribute);
+			assertThat("Response cookie '" + cookieName + "' attribute '" + cookieAttribute + "'",
+					attribute, matcher);
+		};
+	}
+
+	/**
+	 * Assert a cookie's specified attribute.
+	 * @param cookieAttribute the name of the Cookie attribute (case-insensitive)
+	 * @since 6.0.8
+	 */
+	public ResultMatcher attribute(String cookieName, String cookieAttribute, String attributeValue) {
+		return result -> {
+			Cookie cookie = getCookie(result, cookieName);
+			assertEquals("Response cookie '" + cookieName + "' attribute '" + cookieAttribute + "'",
+					attributeValue, cookie.getAttribute(cookieAttribute));
+		};
+	}
+
 
 	private static Cookie getCookie(MvcResult result, String name) {
 		Cookie cookie = result.getResponse().getCookie(name);