ci: compile main sources in coverage_report job

The coverage_report job was producing an effectively empty
jacocoTestReport.xml (3.4KB vs ~1.1MB locally) because no .class files
existed when coverageReportOnly ran — the job checked out source code
and downloaded .exec artifacts, but never compiled. JaCoCo's report
generator skips packages/classes it cannot resolve, so the merged XML
ended up with only <sessioninfo> entries and no <package> elements.

That made coverallsJacoco silently no-op via the
"source file set empty, skipping" branch in CoverallsReporter, so
"Push coverage to Coveralls" reported success without uploading.

Verified by downloading the coverage-report artifact from a recent run
and comparing its XML structure against a local build's report.

Assisted-By: Claude Code

Author: sonus21

.github/workflows/java-ci.yaml

@@ -445,6 +445,9 @@ jobs:
       - name: Set up Gradle
         uses: gradle/actions/setup-gradle@v4
 
+      - name: Compile main sources
+        run: ./gradlew compileJava
+
       - name: Download coverage artifacts
         uses: actions/download-artifact@v4
         with: