test(reporting): unit-test ReportPrecompiler to clear the Sonar coverage gate

#320's backend Sonar gate failed on new_coverage (12.1% < 80%): the new
ReportPrecompiler is a build-time main() and reporting is otherwise covered
only by Oracle ITs, which don't run in the CI unit-coverage job.

Add ReportPrecompilerTest (a plain unit test — the CI test JVM is a JDK, so
javac is present, exactly where the precompile runs):
- compiles a real shipped .jrxml to a sibling .jasper, source left in place;
- no-args -> IllegalArgumentException;
- empty dir -> IllegalStateException (the fail-fast the reviewers asked for);
- missing dir -> throws (no silent no-op).

reporting/ isn't in the jacoco/sonar excludes, so this coverage registers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: gpascucci

backend/src/test/java/ca/bc/gov/nrs/ilcr/reporting/ReportPrecompilerTest.java

@@ -0,0 +1,60 @@
+package ca.bc.gov.nrs.ilcr.reporting;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.io.InputStream;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.StandardCopyOption;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+/**
+ * Unit test for the BUILD-TIME report precompiler. Runs on the test JVM (a JDK, so {@code javac} is
+ * present, unlike the JRE runtime container), which is exactly where the precompile is meant to
+ * run. Pins the fail-fast contract: a build that compiles zero templates must fail rather than ship
+ * an app with no {@code .jasper} to load (PR #320 review, SScholefield/Rylan #3).
+ */
+class ReportPrecompilerTest {
+
+  @Test
+  void compilesJrxmlToSiblingJasper(@TempDir Path dir) throws Exception {
+    // A real shipped template — the same ones the process-classes precompile handles.
+    Path jrxml = dir.resolve("schedule5.jrxml");
+    try (InputStream in = getClass().getResourceAsStream("/reports/schedule5.jrxml")) {
+      Files.copy(in, jrxml, StandardCopyOption.REPLACE_EXISTING);
+    }
+
+    ReportPrecompiler.main(new String[] {dir.toString()});
+
+    Path jasper = dir.resolve("schedule5.jasper");
+    assertTrue(Files.exists(jasper), "expected a sibling .jasper to be produced");
+    assertTrue(Files.size(jasper) > 0, "expected the compiled .jasper to have content");
+    // The .jrxml source is left in place (only the extension is swapped, not replaced).
+    assertTrue(Files.exists(jrxml), "the .jrxml source should remain");
+  }
+
+  @Test
+  void failsWhenNoArguments() {
+    assertThrows(IllegalArgumentException.class, () -> ReportPrecompiler.main(new String[] {}));
+  }
+
+  @Test
+  void failsWhenDirectoryHasNoTemplates(@TempDir Path emptyDir) {
+    // A green build that compiled nothing would surface only as a runtime 500 in the JRE container
+    // (the exact bug the precompile fixes), so an empty compile set must fail the build.
+    String[] args = {emptyDir.toString()};
+    IllegalStateException ex =
+        assertThrows(IllegalStateException.class, () -> ReportPrecompiler.main(args));
+    assertTrue(ex.getMessage().contains("*.jrxml"), "message should name the missing templates");
+  }
+
+  @Test
+  void requiresAnExistingDirectory(@TempDir Path dir) {
+    // A missing reports directory is a hard build error (fail-fast), not a silent no-op.
+    Path missing = dir.resolve("does-not-exist");
+    assertThrows(Exception.class, () -> ReportPrecompiler.main(new String[] {missing.toString()}));
+  }
+}