[#2506] Preserve original required state

[emerardd]

Summary

• initialize originallyRequired from annotated option and positional parameter definitions
• keep programmatic builder calls in sync with the original required state
• let argument groups change only the effective required flag
• render the required marker for options that were actually declared required, with regression coverage for annotated, programmatic, positional, and argument-group cases

Closes #2506.

Validation

Using JDK 21:

• .\gradlew.bat :test --tests 'picocli.HelpTest.testDefaultOptionRenderer_usesOriginalRequiredStateForArgGroupOptions' --tests 'picocli.ModelArgSpecTest.testOriginallyRequiredReflectsAnnotatedRequiredState' --console=plain --configure-on-demand --rerun-tasks (both new tests passed)
• .\gradlew.bat :test --tests 'picocli.HelpTest' --tests 'picocli.ModelArgSpecTest' --console=plain --configure-on-demand --rerun-tasks (301 tests, 0 failures, 1 skipped)
• .\gradlew.bat :compileJava --console=plain --configure-on-demand --rerun-tasks
• git diff --check

The root-module test run completed 2,283 tests: 2,282 passed and 32 were skipped; the only failure was the existing Windows/Jansi environment probe HelpAnsiTest.testAnsiEnabled. Maven/Gradle Plugin Portal TLS handshakes were unavailable in this environment, so the successful local runs used the cached dependencies through a temporary offline init script; CI should provide the authoritative clean-environment result.