Improve JaCoCo coverage with targeted branch/exception-path tests

[Copilot]

Current coverage was ~93% and several defensive/error branches in CDI internals were untested, which limited confidence in extension and proxy behavior reported to JaCoCo/Coveralls. This PR adds focused unit tests to close those gaps without changing production code.

• Coverage gap closure in CDI/MyBatis internals

  • Added CoverageImprovementTest to exercise previously missed branches in:
    • MybatisExtension (invalid producer type, unannotated SqlSessionFactory producer, non-@Mapper path, BeanKey compare/equality/hash paths)
    • CDIUtils (missing SqlSessionFactory resolution failure path)
    • SqlSessionManagerRegistry (unsatisfied factory path)
    • MyBatisBean (default qualifier injection path)
    • SerializableMapperProxy (invocation exception unwrapping)
    • MybatisCdiConfigurationException (message propagation)

• No runtime behavior changes

  • Scope is strictly test additions; library/runtime code remains unchanged.

• Coverage outcome

  • JaCoCo overall moved from ~93% to 97.57% instruction / 98.36% line coverage, improving what Coveralls tracks.

Throwable thrown = assertThrows(IllegalStateException.class, () -> proxy.invoke(null, method, null));
assertEquals("boom", thrown.getMessage());

[coveralls]

coverage: 98.361% (+5.3%) from 93.033% — copilot/improve-test-coverage into master