Add maven-plugin module to the CI gate — red Java reactor slipped past green PR checks

[dmealing]

Problem

mvn clean install was red on main while every PR's CI showed green. The metaobjects-maven-plugin mojo tests (MetaDataGeneratorMojoTest, DocsMojoTest) were failing with:

ERR_RELATIVE_REF_IN_CANONICAL: relative reference '::Apple' on @objectRef in file
[mojo/mojo-test-metadata.json] is not allowed in canonical JSON

Root cause: the FR-032 ERR_RELATIVE_REF_IN_CANONICAL guard (commit af46e655) started rejecting relative :: refs in canonical JSON, but the maven-plugin test fixture mojo-test-metadata.json still used @objectRef: "::Apple" / "::Orange". The maven-plugin mojo tests are not part of the PR conformance/CI gate, so the break was invisible until a local full-reactor mvn clean install during the 7.3.0 release.

Fixed in release commit a9a962d0 (refs qualified to simple::fruitbasket::Apple/Orange).

Fix

Add the metaobjects-maven-plugin module (and ideally the full mvn -pl <plugin> test or a full-reactor mvn install) to the Java CI workflow / branch-protection required checks, so a red reactor can't slip past green PR checks again. Consider the same audit for any other reactor module not currently exercised by CI (e.g. fatjar-smoke, integration-tests*).

[dmealing]

Fixed in #82 (in CI). For #37: branch protection must add the required check conformance / java-reactor (job renamed).