[18.0][FIX] account_manual_currency: compute invoice_currency_rate and test script

[Saran440]

• Fixed line balance not calculating with manual currency.
• Fixed payment move not using manual currency.
• Fixed test script.

[ThiagoMForgeFlow]

Why do you remove the previous logic? I mean The test added and changes from the payment wizard

[Saran440]

For this line https://github.com/ForgeFlow/account-invoicing/pull/3/files#diff-e05d77639f1fb23e46330d6c3fddc9754b6ae4d1267efc0c47789041420168b1R155

• When we set manual_currency_rate to 0, it automatically changes to 1 instead of staying at 0.
• I think it should reset to the default currency instead.

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For this line https://github.com/ForgeFlow/account-invoicing/pull/3/files#diff-e05d77639f1fb23e46330d6c3fddc9754b6ae4d1267efc0c47789041420168b1L158

• This line should be deleted because we can't create an invoice with manual_currency_rate = 0.
• Therefore, this method is not needed.

[ThiagoMForgeFlow]

I agree with the new changes but, why do you remove the test added before to check the functionality with the payments?
OCA@e198e88#diff-fd90b3a59da8e13a98203d8d4f4259bf6aee70071a2d2c00ef4b74efbf2ef2e0R139

[Saran440]

Sorry, I thought this PR had already been merged,
so I rebased and dropped the old commits, which caused the test script that was added to be lost.

[ThiagoMForgeFlow]

Okay, I just added this commit to the original PR.