Increase decimal points in order POST-PUT requests to mitigate tax rounding issues

[iamgabrielma]

This is the iOS counterpart of woocommerce/woocommerce-android#14134

Description

This PR fixes order creation rounding issues by adding the dp=8 (decimal places) parameter in create and update order API requests, which by default is set to 2 if nothing is passed in.

This increases decimal precision to reduce discrepancies between client-side calculations and backend totals, addressing cases where item totals were incorrectly rounded (e.g., $6 product with 9% tax showing $11.00 instead of $11.01 for quantity of 2).

Changes

• Updated decimal points from 2 to 8
• Updated a variable name, just for clarity.
• Added regression tests

Steps to reproduce/Testing information

In general I found the tax setup and results quite confusing when sending from one country to another, the easier setup seemed to be to set your store to country X, and sells to a customer of the same country. Here's some conversation with some troubles we found during the debugging process in case you find something off with the tax calculation: p1753783743005429-slack-CGPNUU63E

• Add a product with price that results in non-terminating decimal after tax calculation (e.g., $6 with 9% tax = $5.504587 pre-tax) (Make sure the Enter product prices including tax settings is enabled)
• Increase quantity to 2 or more in cart
• Observe total calculation discrepancy between displayed rounded values and actual precise calculations
• Complete order creation and verify totals match between app and WooCommerce backend

Screenshots

Before	After

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

[wpmobilebot]

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

	App Name	WooCommerce iOS Prototype
	Build Number	pr15957-1c5234a
	Version	22.9
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	1c5234a
	Installation URL	6vg9bt0c5kq7g

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[staskus]

@iamgabrielma excuse me for not reaching it today, I will review it tomorrow.

[staskus]

Nice, great work and investigation. Taxes are always a challenging area.

I didn't know we can send more decimal points to the API. I was always frustrated with some of the imprecision that would happen with a combination of tax rates and product prices. This does look to improve it 👍