Add testing skill for the Java 17 / Spring Boot 3.x app

.agents/skills/testing-springboot-java17/SKILL.md

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+---
+name: testing-springboot-java17
+description: Build, run and end-to-end test the gs-spring-boot demo app (Java 17 / Spring Boot 3.x) on this box, including working around Maven Central rate limiting.
+---
+
+# Testing the springboot-java8 (now Java 17 / Spring Boot 3.x) app
+
+## Build
+
+- Java 17 is the default JDK (`java -version` -> 17.x). No toolchain install needed.
+- **Maven**: `./mvnw` may fail — the wrapper script is often not executable (`Permission denied`) and
+  downloading `apache-maven-*.zip` from `repo1.maven.org` can return **HTTP 429** (rate limited).
+  Workaround: use the system Maven, which picks up the google mirror in `~/.m2/settings.xml`:
+  `mvn clean package` -> `target/gs-spring-boot-0.1.0.jar` (boot-repackaged).
+  If you must use the wrapper, run it as `sh mvnw` and expect the 429 unless a local dist is cached.
+- **Gradle**: `build.gradle` declares `mavenCentral()`, which hits the same rate limit. Use an init
+  script that swaps the repo for the Google mirror:
+  ```groovy
+  // /tmp/mirror.gradle
+  allprojects {
+      afterEvaluate {
+          def kept = repositories.findAll { !(it.hasProperty('url') && it.url != null && it.url.host in ['repo.maven.apache.org','repo1.maven.org']) }
+          repositories.clear()
+          repositories.maven { url = 'https://maven-central.storage-download.googleapis.com/maven2' }
+          kept.each { repositories.add(it) }
+      }
+  }
+  ```
+  Then `./gradlew clean build -I /tmp/mirror.gradle` -> `build/libs/gs-spring-boot-0.1.0.jar`.
+  Note the init script only rewrites project repositories, not `buildscript {}` ones, so a cached
+  Gradle dependency cache may still be required; if the buildscript block fails, add the same
+  mirror inside `buildscript { repositories { ... } }` temporarily (do not commit).
+
+## Run
+
+`cd <repo> && java -jar target/gs-spring-boot-0.1.0.jar > /tmp/app.log 2>&1 &` — port 8080
+(`application.properties` has `server.port` commented out). Run from the repo root: the
+`/topic/file/operation` endpoint reads `temp.txt` from the process CWD.
+
+Healthy startup log contains: `Starting Servlet engine: [Apache Tomcat/10.1.x]`, `Started Application`,
+four `Inserting customer record for ...` lines and two `Customer{... firstName='Josh' ...}` lines
+(proves the JdbcTemplate varargs `query(...)` overload works on Spring 6), plus a **WARN** (not an
+error) for the unreachable `http://gturnquist-quoters.cfapps.io/api/random` quote URL — that domain
+no longer resolves, so a WARN is expected behaviour.
+
+## Endpoints (all `@RestController`, JSON/plain text, no auth)
+
+| Route | Notes |
+|---|---|
+| `GET /` (`?name=X`) | GreetingController, counter increments each call |
+| `GET /topic`, `GET /topic/{id}` | TopicController |
+| `POST /topic`, `PUT /topic/{id}`, `DELETE /topic/{id}` | return 200 with empty body (void handlers) |
+| `GET /topic/minimum/length/{n}` | ids with length **>** n |
+| `GET /topic/sort` | sorts the in-memory list in place |
+| `GET /datetime`, `GET /topic/string/operation`, `GET /topic/file/operation` | Java 8 stream/NIO demos |
+
+State is a static in-memory list, so mutations persist until restart — restore state (delete what
+you POSTed) or restart the jar between runs.
+
+## Known pre-existing behaviour (not migration fallout)
+
+- `GET /topic/{unknown-id}` -> **500** whitelabel error page; `TopicService.getTopicWithId` calls
+  `Optional.get()` without a presence check. Expected 404 would need a code fix.
+- `PUT`/`DELETE` on a non-existent id -> 200 no-op.
+
+## Devin Secrets Needed
+
+None — the app is fully local with an in-memory H2 database.