[in_app_purchase] Add IMMEDIATE_AND_CHARGE_FULL_PRICE to ProrationMode

[tosaka07]

Added IMMEDIATE_AND_CHARGE_FULL_PRICE to ProrationMode.

IMMEDIATE_AND_CHARGE_FULL_PRICE is new in Billing Library v4. https://developer.android.com/google/play/billing/release-notes#4-0-0-summary-changes

This library has already been updated to v5, so there is no problem.

Fixes https://github.com/flutter/flutter/issues/108164

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [x] I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• [x] I signed the CLA.
• [x] The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• [x] I listed at least one issue that this PR fixes in the description above.
• [x] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• [x] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• [x] I updated/added relevant documentation (doc comments with ///).
• [ ] I added new tests to check the change I am making, or this PR is test-exempt.
  • just added the enum case.
• [x] All existing and new tests are passing.

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[flutter-dashboard[bot]]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

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[stuartmorgan-g]

Thanks for the contribution! You’ve checked boxes in the PR checklist above that are not reflected in this PR, so I’m assuming this is a work in progress and am marking it as a Draft. Please review the checklist, updating the PR as appropriate, and when the checklist reflects the state of the PR as posted please feel free to mark it as ready for review.

[tosaka07]

@stuartmorgan Sorry about that. I have reviewed the checklist and updated CHANGELOG and pubspec.

ProrationMode is mapped to Int by JsonSerializable. Therefore, I thought no additional test was needed, is that correct?

[stuartmorgan-g]

ProrationMode is mapped to Int by JsonSerializable. Therefore, I thought no additional test was needed, is that correct?

As it says in the automated response text above:

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Would it matter to you if this enum value were accidentally dropped in a later update? Or if the value were accidentally changed to something other than 5 such that it didn't work correctly?

[GaryQian]

https://github.com/flutter/plugins/pull/6415 is based off of this and has landed. @tosaka07 Thank you for your contribution!