[PM-23702] Feature/billing controller tests

[Alexander-Infante]

🎟️ Tracking

Additional unit testing for the AppleController and StripeController in the Billing modules.

📔 Objective

Adding additional unit testing for the Billing Controller module, specifically for Apple IAP and Stripe. These are net new testing suites that are additive and focused on testing invalid inputs to ensure they are handled properly within their respective controllers.

⏰ Reminders before review

• Contributor guidelines followed
• All formatters and local linters executed and passed
• Written new unit and / or integration tests where applicable
• Protected functional changes with optionality (feature flags)
• Used internationalization (i18n) for all UI strings
• CI builds passed
• Communicated to DevOps any deployment requirements
• Updated any necessary documentation (Confluence, contributing docs) or informed the documentation team

🦮 Reviewer guidelines

• 👍 (:+1:) or similar for great changes
• 📝 (:memo:) or ℹ️ (:information_source:) for notes or general info
• ❓ (:question:) for questions
• 🤔 (:thinking:) or 💭 (:thought_balloon:) for more open inquiry that's not quite a confirmed issue and could potentially benefit from discussion
• 🎨 (:art:) for suggestions / improvements
• ❌ (:x:) or ⚠️ (:warning:) for more significant problems or concerns needing attention
• 🌱 (:seedling:) or ♻️ (:recycle:) for future improvements or indications of technical debt
• ⛏ (:pick:) for minor or nitpick changes

[bitwarden-bot]

Thank you for your contribution! We've added this to our internal Community PR board for review.
ID: PM-23702
Link: https://bitwarden.atlassian.net/browse/PM-23702

Details on our contribution process can be found here: https://contributing.bitwarden.com/contributing/pull-requests/community-pr-process.