Adds a `readme_examples.dart` file under `example/lib` for the `in_app_purchase` package and switches the package README to use `<?code-excerpt?>`-generated snippets sourced from it, instead of hand-maintained inline code blocks. This keeps the README examples compiling and in sync with the current package API, and removes `in_app_purchase`'s exemption from `code-excerpt` adoption.

Also fixes several runtime-safety issues in those new excerpts that were flagged in review (missing `Platform.isIOS`/`Platform.isMacOS` guards around StoreKit-only APIs, and unsafe null/index access when reading Android subscription offers and StoreKit 2 transactions), and bumps the package version with a CHANGELOG entry for the change.

Addresses the `in_app_purchase/in_app_purchase` item of flutter/flutter#102679 (that issue tracks many packages, so it should stay open after this merges).

## Pre-Review Checklist

Test plan

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