fix InvoiceItem.UnitPrice type

[jwhited]

It's a string, not a float. A float makes sense, but I only get strings
back on the wire.

Fixes #848

[Copilot]

Pull Request Overview

This PR fixes a type mismatch in the InvoiceItem.UnitPrice field, changing it from float32 to string to match the actual API response format.

• Changed InvoiceItem.UnitPrice field type from float32 to string
• Updated test fixture and test assertion to reflect the corrected type

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
account_invoices.go	Changed UnitPrice field type from float32 to string in the InvoiceItem struct
test/unit/fixtures/account_invoice_items_list.json	Updated test fixture to represent unit_price as a string value instead of a number
test/unit/account_invoices_test.go	Updated test assertion to compare UnitPrice against a string value instead of float32

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[Copilot]

The InvoiceItem struct has inconsistent numeric types. While UnitPrice is now correctly a string to match the API response, other monetary fields (Amount, Tax, Total) remain as float32. Consider whether these fields also return as strings from the API. If they do, they should be changed to string for consistency. If they're actually returned as numbers, the inconsistency should be documented to explain why UnitPrice is treated differently.