Add kotlin module: null safety, coroutines, allopen/noarg, JSpecify interop (post #42)

kotlin/docs/output/09-no-testresttemplate.txt

@@ -0,0 +1,25 @@
+Real, verified absence: Spring Boot 4.1.1's spring-boot-test module does not
+ship org.springframework.boot.test.web.client.TestRestTemplate at all.
+
+Resolved test-scope dependency tree (mvn dependency:tree), relevant slice:
+
+[INFO] +- org.springframework.boot:spring-boot-starter-test:jar:4.1.1:test
+[INFO] |  +- org.springframework.boot:spring-boot-test:jar:4.1.1:test
+[INFO] |  +- org.springframework.boot:spring-boot-test-autoconfigure:jar:4.1.1:test
+...
+[INFO] |  +- org.springframework:spring-test:jar:7.0.9:test
+
+Checked directly against the jar itself (not docs, not a blog post):
+
+    $ unzip -l spring-boot-test-4.1.1.jar | grep -i "resttemplate\|restclient\|webtestclient"
+    (no output -- none of those classes are in the jar)
+
+Confirmed via javac/kotlinc failing to resolve it: a first draft of
+BookControllerTest.kt that imported
+org.springframework.boot.test.web.client.TestRestTemplate failed to compile with
+"Unresolved reference 'TestRestTemplate'" and "Unresolved reference
+'getForEntity'" -- not a missing dependency, a genuinely absent class.
+
+This repository's tests use plain java.net.http.HttpClient against
+@LocalServerPort instead. The direct replacement in Spring Framework 7 is
+RestTestClient, covered in its own post.