feat: expose sub_account_name on Number resource

[avinashp-plivo]

Summary

• Adds sub_account_name to the Number#to_s output so it shows up alongside sub_account when the resource is printed.
• The attr_reader for @sub_account_name is already created dynamically by Base::Resource#parse_and_set once the API returns the field, so this is purely a to_s parity tweak.
• Mock responses (numberGetResponse.json, numberListResponse.json) updated to include sub_account_name; the spec's to_json helper updated to mirror.

Mirrors the parallel work on plivo-java#317, plivo-php#371, plivo-dotnet#312 for FNT-370.

Test plan

• bundle exec rspec spec/resource_numbers_spec.rb passes locally (5/5)
• Updated mocks contain the new field for both get and list cases

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