[in_app_purchase_storekit] Add support for quantity in consumable product purchases (#171570)

[nikunjrCybage]

This PR adds support for specifying a quantity when purchasing consumable products using StoreKit 2 in the in_app_purchase_storekit package.

✅ What’s changing

• Introduces logic to read the quantity field from PurchaseParam and insert it into the native StoreKit 2 .purchase(options:) call.
• Ensures this logic is only applied to consumable products.
• Adds null checks and default behavior to maintain compatibility with purchases that don’t specify quantity.

🧪 Testing

• Added unit tests to verify quantity handling in Sk2PurchaseParam:
  • ✅ Verifies that a custom quantity is passed correctly to native purchase options.
  • ✅ Ensures that quantity defaults to 1 when not explicitly provided.
• Tests cover buyConsumable flows using fakeStoreKit2Platform.
• Verified on a real device using StoreKit 2 with different quantity values.
• Ensures no change to StoreKit 1 flow or other product types.

📌 Fixes

Fix #171570

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[gemini-code-assist]

Code Review

This PR adds support for specifying a quantity when purchasing consumable products with StoreKit 2. The implementation is straightforward, but it's missing crucial validation to ensure the quantity is only used for consumable products and that its value is positive, as required by Apple's StoreKit framework. I've suggested adding guards to handle these cases.

[LongCatIsLooong]

Thanks for the fix! The patch itself LGTM, but you may have to:

1. update the PR description a bit so when it gets merged it also closes the associated issue (basically you want to replace the markdown link with the issue url after the "fix" keyword)
2. The version of the plugin needs to be bumped and you'll need to add a simple description of this change to the CHANGELOG file.

[LongCatIsLooong]

LGTM

[Bradley-McCallion]

Looks good to me too! @LouiseHsu is there anything else needed to get this in?

[LongCatIsLooong]

CI failed on a format issue:
https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8707767920550695297/+/u/Run_package_tests/format/stdout

[nikunjrCybage]

@LongCatIsLooong Thanks for pointing that out! I've fixed the formatting issue—CI should pass now. ✅

[LongCatIsLooong]

still LGTM.

[LouiseHsu]

LGTM also!