fix(rest): separate resource errors from base errors

[efegokdemir]

Description

Fixes #1255

Generated DiscountCode resources define an API errors field, which shared the @errors instance variable used by Rest::Base for HTTP error tracking. Keep the base error collection separate so the resource field retains its declared value and failed saves re-raise the original HttpResponseError.

How has this been tested?

• The regression test failed before the fix with Sorbet TypeError when reading DiscountCode#errors; it passes after the fix.
• bundle exec ruby -Itest test/rest/base_errors_test.rb — 4 tests passed.
• API_VERSION=2023_10 bundle exec rake test:rest_wrappers — 642 tests passed.
• API_VERSION=2026_07 bundle exec rake test:rest_wrappers — 626 tests passed.
• bundle exec rake test:rest_wrappers — 10,883 tests passed.
• bundle exec rake test:library — 342 tests passed.
• bundle exec rubocop — 108 files inspected, no offences.

srb tc could not run because the locked Sorbet binary is x86_64 and cannot execute on this host.

Checklist:

• My commit message follow 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.