chore: Add type hints to Chargebee functions

[khvn26]

Thanks for submitting a PR! Please check the boxes below:

• I have read the Contributing Guide.
• I have added information to docs/ if required so people know about the feature.
• I have filled in the "Changes" section below.
• I have filled in the "How did you test this code" section below.

Changes

In this PR, we add type hints to our Chargebee code so it's slightly more pleasant to work with.

How did you test this code?

The changes are non-functional. All existing tests pass. Newly added explicit return None branches are covered by new tests.

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

3 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
docs	Ignored	Preview	Feb 26, 2026 4:32pm
flagsmith-frontend-preview	Ignored	Preview	Feb 26, 2026 4:32pm
flagsmith-frontend-staging	Ignored	Preview	Feb 26, 2026 4:32pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-frontend:pr-6790	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-6790	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-6790	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6790	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-e2e:pr-6790	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-6790	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-6790	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-6790	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api-test:pr-6790	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6790	Finished ✅	Results ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.30%. Comparing base (9a09772) to head (e3ff982).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6790      +/-   ##
==========================================
+ Coverage   98.28%   98.30%   +0.01%     
==========================================
  Files        1330     1331       +1     
  Lines       49255    49332      +77     
==========================================
+ Hits        48412    48494      +82     
+ Misses        843      838       -5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.