[Woo POS] [Cash & Receipts] Validate cash amount

[iamgabrielma]

Closes: #14749
#14602 must be merged first

Description

This PR adds validation to the cash amount entered by a merchant during the cash collection flow.

• If the amount is not enough, does not show any message until we press the "collect" button, at which point an error message (red) will appear
• If the amount is exact, we can collect normally
• If the amount exceeds the order total, a "change due" (green) message will be shown each time we change the input.

Screen.Recording.2025-01-07.at.19.19.53.mov

Testing

• Enable .acceptCashForPointOfSale feature flag
• In POS, add items to the cart, then tap on "Cash Payment"
• Enter an amount less, equal, and exceeding the order total. Observe that responds as in the description
• Using . or , for decimals should behave the same.
• Upon completion, the order appears in the app under the Orders tab.

You will see some additional space at the bottom of the success view, as well as the floating button to connect the reader. This is unrelated to this PR and will be addressed in the parent PR (#14602) or in a follow-up

---

• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

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[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce iOS by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS
	Build Number	pr14811-1339f75
	Version	21.3
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	1339f75
	App Center Build	WooCommerce - Prototype Builds #12385

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[iamgabrielma]

I've attempted to implement this from zero using Decimal but dealing with currency was anything but fun, so I've relied on our own CurrencyFormatter implementation. That said we seem to carry our own downsides (eg: #13829 ) so I'm not sure if there's a better way we could use to handle formatting here, since we use the same currency formatter in other places in POS 🤔

[staskus]

Yes, understandable. It's okay to use what we currently have. We can make improvements internally within the formatters.

The largest issues I've seen were when converting amounts back and forth when creating and then editing Order fields again. It's not the case with this POS cash payment so it should be fine.

[staskus]

Works well, thanks!

I suggest moving cash calculations into a separate helper and to write unit tests.

[staskus]

Yes, understandable. It's okay to use what we currently have. We can make improvements internally within the formatters.

The largest issues I've seen were when converting amounts back and forth when creating and then editing Order fields again. It's not the case with this POS cash payment so it should be fine.

[staskus]

All these methods could be unit tested (with mocked currency formatter). We have calculations happening within them. These methods would likely need to change into pure functions within a helper and then View would just set appropriate labels based on the result.