Preserve nested REST attribute changes on save

[efegokdemir]

Description

Fixes #1331

create_instance kept mutable nested response values by reference in original_state, so in-place edits also changed the comparison baseline and could be omitted from save requests. This deep-copies the baseline and adds a regression test.

How has this been tested?

• bundle exec rake test:library — 343 tests passed.
• API_VERSION=2026_07 bundle exec rake test:rest_wrappers — 626 tests passed.
• Targeted RuboCop passed.
• The regression test failed before the fix and passed after it. The all-version wrapper suite was not completed.

Checklist:

• My commit message follows the pattern described in here
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the project documentation.
• I have added a changelog line.