refactor: migrate Xtend to Java - com.avaloq.tools.ddk.checkcfg.*

[joaodinissf]

Migrates all 11 Xtend files in the checkcfg.* bundles to Java 21, taking the bundle family fully off Xtend. One commit per module.

com.avaloq.tools.ddk.checkcfg.core (4 files)

CheckCfgGenerator, CheckCfgJvmModelInferrer, PropertiesInferenceHelper, ConfiguredParameterChecks converted; .xtend originals deleted; xtend-gen removed from .classpath/build.properties. The bundle keeps its Xtext nature (it hosts the CheckCfg.xtext grammar).

com.avaloq.tools.ddk.checkcfg.core.test (7 files)

CheckCfgContentAssistTest, CheckCfgScopeProviderTest, CheckCfgSyntaxTest, CheckCfgModelUtil, CheckCfgTestUtil, CheckCfgConfiguredParameterValidationsTest, CheckCfgTest converted; full infrastructure cleanup including the now-unused org.eclipse.xtend.lib manifest entry and the Xtext builder/nature in .project.

com.avaloq.tools.ddk.checkcfg.ide

Pure scaffolding removal: the bundle never had Xtend sources, but carried xtend-gen entries in .classpath/build.properties and an empty xtend-gen directory.

Intentional deviations from the Xtend compiler output (all behavior-equivalent)

• Test methods declare throws Exception instead of Xtend's generated sneakyThrow wrapping.
• CheckCfgScopeProviderTest throws IllegalStateException where the Xtend source threw a raw Exception and an explicit NullPointerException (static-analysis-clean, same failure semantics); local constants renamed to camelCase per Java naming rules.
• Templates follow the string-building conventions: text blocks for static sources, .formatted() for interpolation without control flow, StringBuilder only where there is loop/conditional logic. String outputs match the Xtend compiler output exactly, including trailing newlines.
• PropertiesInferenceHelper.inferType uses an exhaustive Java 21 switch expression over the enum, with the original null-tolerance preserved (a null PropertyType still yields null, as in Xtend's switch semantics).
• Stdlib collections replace Guava where the consumer only iterates (new ArrayList<>(0), new HashMap<>()); Sets.newHashSet(array) kept where Guava is genuinely more concise.
• A malformed Javadoc tag (@param parameterto check.) is minimally corrected to @param parameter to check.

Verification

Full mvn verify with Checkstyle and PMD: all 10 checkcfg test methods pass (the only suite failure is the known local-machine-only testBulkApplyingQuickfix SWTBot flake in an untouched bundle, which is consistently green in CI).

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