Add JUnit 4 unit tests for TopicService

[devin-ai-integration]

Summary

Adds the project's first tests: src/test/java/hello/service/TopicServiceTest.java, 15 JUnit 4 characterization tests covering the deterministic, in-memory logic of TopicService (seeded ids spring, java, javascript). Each test constructs new TopicService() in @Before, so they document current behavior without touching production code.

Coverage: getAllTopics, getTopicWithId (incl. NoSuchElementException for an unknown id — documents the no-fallback .findFirst().get()), addTopic, updateTopic (replace + no-op for unknown id), deleteTopic (remove + no-op), filterMinimumLengthForId (strict > + boundaries), sortTopicsWithID, returnAllTopicIDWithStringSlicing ("spring:java:javascript"), makeDistinctAndSortCharacters ("cba"→"abc", "banana"→"abn"), splitAllIdWithColonSelectIDWithJavaKeywordThenSortThenJoin ("java:javascript"), and findIdHavingCharacter ("[spring]").

Build wiring:

• pom.xml: add spring-boot-starter-test (test scope).
• build.gradle: replace testCompile("junit:junit") with testCompile("org.springframework.boot:spring-boot-starter-test").
• build.gradle also gained spring-boot-starter-jdbc + com.h2database:h2 (already present in pom.xml). These are required — without them Gradle's compileJava fails (Application.java imports org.springframework.jdbc.core.JdbcTemplate), which blocked compiling/running any test.

Verification

./gradlew test → BUILD SUCCESSFUL, 15 tests, 0 failures (run with JDK 8, matching Spring Boot 2.0.2 / Gradle 4.6).

Note: pom.xml uses <packaging>pom</packaging>, so mvn test binds no compile/surefire and executes no tests — the spring-boot-starter-test dep is wired in as requested, but Gradle is the effective test runner here.

Link to Devin session: https://app.devin.ai/sessions/7d9ce4715bd240c689e7fe5259a0e560
Requested by: @patrickbradley-cog

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[devin-ai-integration]

Test results — all 15 tests pass ✅

Ran the full suite ./gradlew clean test (JDK 8 / Gradle 4.6 / Spring Boot 2.0.2) and verified via the JUnit XML and generated HTML report.

• :test ran and printed BUILD SUCCESSFUL — passed
• Results XML: tests=15 skipped=0 failures=0 errors=0 — passed
• Gradle HTML report: 15 tests, 0 failures, 100% successful — passed

Gradle HTML test report (15/15 passed)

Terminal: BUILD SUCCESSFUL + results XML count

Note: mvn test executes zero tests here because pom.xml uses <packaging>pom</packaging> (no surefire binding); Gradle is the effective test runner.

Tested by Devin: https://app.devin.ai/sessions/7d9ce4715bd240c689e7fe5259a0e560