[in_app_purchase] Fix broken dartdoc references in completePurchase

[ishaquehassan]

The dartdoc on InAppPurchase.completePurchase and InAppPurchasePlatform.completePurchase referenced a PurchaseException class and a PurchaseException.errorCode field, neither of which exists in this package. The actual class thrown by completePurchase is InAppPurchaseException and its error code field is named code. Because of the typo the dartdoc links rendered as broken on pub.dev and api.flutter.dev.

This PR rewrites the doc comment in both files to point at InAppPurchaseException, InAppPurchaseException.code, and InAppPurchaseException.message so the references resolve. The platform interface file also gains an errors/errors.dart import so the class is in scope for the dartdoc link. No runtime behavior changes.

Fixes flutter/flutter#118626

Pre-Review Checklist

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• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹. Test-exempt: this PR only corrects documentation text, with no behavioral changes.
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Footnotes

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests or get an explicit test exemption before merging.

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[gemini-code-assist]

Code Review

This pull request corrects dartdoc references in the completePurchase method across the in_app_purchase and in_app_purchase_platform_interface packages, replacing references to the non-existent PurchaseException and errorCode with InAppPurchaseException and code. It also adds a necessary import to the platform interface and updates the changelogs for both packages. I have no feedback to provide.

[stuartmorgan-g]

• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under1.

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 state of the PR as posted reflects the checklist please feel free to mark it as ready for review.