feature: add JUnit test suite and JaCoCo coverage reporting

[devin-ai-integration]

Summary

The repo had no tests at all, and couldn't have had any: pom.xml declared <packaging>pom</packaging>, so Maven never compiled src/main/java, let alone a test source root. This adds 41 JUnit 4 tests covering every class under hello.* (90% line / 80% branch per JaCoCo) plus a coverage report.

Build changes (all in pom.xml, no src/main behavior touched):

• packaging: pom -> jar — the prerequisite for compiling and testing anything.
• spring-boot-starter-test (test scope) and jacoco-maven-plugin 0.8.8 (prepare-agent + report bound to verify, output at target/site/jacoco/).
• Pins needed to run tests on a modern JDK, since the Spring Boot 2.0.2 defaults predate it: maven-surefire-plugin 2.22.2, mockito.version 4.11.0, byte-buddy.version 1.14.9.

Test notes worth knowing before reading the diff:

• Controllers use @WebMvcTest(X.class) with @ContextConfiguration(classes = X.class) and a @MockBean TopicService. The narrow context is deliberate: loading Application would drag in its JdbcTemplate autowire and the CommandLineRunner that calls the (long-dead) gturnquist-quoters.cfapps.io quote API.
• ApplicationTest exercises Application.run() for real against an EmbeddedDatabaseBuilder H2 datasource assigned to the package-private jdbcTemplate field, asserting the 4 seeded customers and the 2 first_name = 'Josh' rows — no @SpringBootTest, so no network.
• TopicService's NIO methods resolve Paths.get("") against the process CWD, which under Surefire is the project basedir. The assertions therefore lean on files that are committed: pom.xml for findAllFilesInPathAndSort(), gradlew for the "grad" prefix search, and the repo's temp.txt for readFileWithStreamFunction() == " Hello, this, is, Rehman". Running these tests from a different working directory would fail them.
• SimpleTimeClient.setDate(int day, int month, int year) delegates to LocalDate.of(day, month, year), i.e. the parameter names are in the wrong order. Left as-is (out of scope); the test calls it as setDate(2020, 5, 15) and asserts against the actual behavior rather than the names.

mvn clean verify passes with 41 tests, 0 failures, 0 skips.

Devin-Org: engineering

Link to Devin session: https://app.devin.ai/sessions/4c5fa48440e44d848e7d5554fbda3d8f
Requested by: @shayanshafii

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