[VDG] Simplify insufficient balance handling

[soosr]

#9289 simplified a lot so the unused code can be removed.

I started with only removing the dead code but ended up with refactoring. So from now, only fee rate change can cause the exception. Previously it could be due to Pocket changing, total balance sending, and fee changing.

What is the new logic:
The user increases the transaction fee, selected amount cannot cover it anymore,

1. Always select more coins (in the background) until it is possible.
2. If not possible, then adjust the transaction fee to the maximum possible one.
3. If not possible, then error.

Test cases:

• Send your total balance (fee is subtracted from the amount)
• Send a little bit less than your total balance (error)
• Use the lowest fee rate, send a little bit less than the total balance, change the fee rate to the maximum. (fee rate should be adjusted)
• Low fee rate. Send an amount which little bit less than pocket A amount, (transaction uses pocket A). Increase fee. (additional pockets should be selected to cover the fee)

[soosr]

@yahiheb could you test this PR?

[soosr]

Converted to draft, found a case that is not covered with the new solution.

[soosr]

Ready for test and review @zkSNACKs/visual-design-group @yahiheb

[ichthus1604]

tACK. Code is also okay, with minimal style observations in classes such as TransactionPreviewViewModel, which are not directly related to this PR and hence will be treated separately.

[SuperJMN]

utACK.

Code looks fine. The BuildTransactionAsync method is now more readable. Good work!
Please, see the comments regarding future improvements.