Run common tests from build script and tighten localization packaging/resource handling

[shai-almog]

Motivation

• Ensure localization resources are present and packaged correctly for generated projects and enable running the relevant common-module tests during local builds.
• Replace ad-hoc assumptions about specific language bundles with a data-driven handling based on ProjectOptions.PreviewLanguage.

Description

• Added INITIALIZR_RUN_TESTS support and RUN_TESTS_DEFAULT to scripts/initializr/build.sh, with helper functions should_run_tests and run_common_tests to optionally run the common module tests before platform-specific packaging steps.
• Constrained the tests invoked from the build script to GeneratorModelMatrixTest and GeneratorModelLocalizationPackagingTest via -Dtest=... to validate localization packaging behavior.
• In GeneratorModel, introduced readRequiredResourceToString and addLocalizationEntry and switched localization file insertion to use these helpers so missing required resources now raise an IOException rather than silently returning null.
• Updated GeneratorModelMatrixTest to assert localization bundles via iteration over ProjectOptions.PreviewLanguage instead of hardcoding languages, and added a new GeneratorModelLocalizationPackagingTest that compiles a dummy class and verifies generated localization resources load via a classloader.

Testing

• Ran the selected common module tests with Maven using ./mvnw -pl common -am test -DskipTests=false -DfailIfNoTests=false -Dtest=GeneratorModelMatrixTest,GeneratorModelLocalizationPackagingTest and both tests completed successfully.
• The new packaging test compiles a temporary class, writes generated localization resources, and verifies resource resolution via a URLClassLoader, and it passed in the automated run.

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Codex Task

[github-actions]

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