Modernize property code generation and validate generated Java 🤖

[lwrage]

Fixes #3237

Summary

The property generator used Xtend, had no Maven build, and tested its generated Java only through source-string comparisons. Convert all 27 Xtend sources in the generator, UI handler, and tests to idiomatic Java 21 with JDK collections, and remove obsolete Xtend/Guava collection dependencies and Eclipse wiring.

Modernize the emitted Java with var, pattern matching, expression lambdas, Optional.isEmpty(), and Stream.toList(). Generated blank lines are empty. Add POMs for the three bundles and a tools profile in both aggregators so normal root builds continue to exclude the tools.

All 26 existing exact-output tests now compile their generated sources and imported property sets with --release 21. Six additional tests use the validated models/generatedjava AADL project to load and execute freshly compiled classes, checking:

• Scalar getters, constant/property references, missing and inapplicable values, defaults, and mode selection.
• Nested lists and rejection of mutations in property, constant, and record-field lists.
• Range bounds/delta, imported enumeration and unit conversion, and nested record extraction, equality, hashing, and round-tripping.

The tests use standard Eclipse Xtext testing APIs. Temporary generated source/classes and class loaders are cleaned up after each test.

Validation

mvn -o -T5 -s releng/osate.releng/settings.xml -f tools/pom.xml -Plocal,tools \
  -Dtycho.localArtifacts=default -Dpr.build=true -Dsign=false \
  -Dspotbugs=false -Dcodecoverage=false -Djavadoc=false \
  clean verify

• Four-project tools reactor passed: 32 tests, zero failures, errors, or skips.
• Generated-source compilation and runtime tests passed on JDK 21.
• git diff --cached --check passed.
• Validation was limited to the requested tools profile; the full root reactor was not used for final PR validation.

Compatibility and dependencies

Generated code targets Java 21. The deliberate behavior change is that Stream.toList() returns unmodifiable lists, including nested lists; callers that need to mutate them must make mutable copies. Exact-output expectations and runtime assertions cover this change. The UI handler compiles, but interactive UI behavior was not exercised.

Based directly on master, with no prerequisite PR or merge-order dependency. The migration is kept in one commit because the source ownership, build wiring, and updated output expectations form one coherent change.