Prevent underflow in Amount

[robinbb]

@Isaac-DeFrain Please have a look at the changes in this PR.

Note that, as-is, it passes tier-1 and tier-2 tests. Tier-3 fails like so:

2025-04-11T19:37:18.102140-07:00 - ERROR Underflow in Sub<Amount>: self.0: 0 rhs.0: 1000       
                                                                                               
thread 'tokio-runtime-worker' panicked at src/ledger/account/mod.rs:259:17:                    
Underflow in Account::debit. pk: B62qix9vooX5NqJYo8nT6xWqCeQu5AJoS1ng6FRnUpVAra6PAZZ1CU4 balance: 0 amount: 0.000001
stack backtrace:                                                                               
   0:     0x5b51de66a129 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::hfc616348d9ad0abc
   1:     0x5b51de0419f3 - core::fmt::write::h7ca648217bc79799                                                                                                                                
   2:     0x5b51de6696c2 - std::io::Write::write_fmt::h7960c58bfa5ccbcb                        
   3:     0x5b51de669f83 - std::sys::backtrace::BacktraceLock::print::h3fb349e80cbe0423                                                                                                       
   4:     0x5b51de669339 - std::panicking::rust_panic_with_hook::h66e909d048c263a9                                                                                                            
   5:     0x5b51de6a7758 - std::panicking::begin_panic_handler::{{closure}}::h8d9aa8be7e8634cf 
   6:     0x5b51de6a76b9 - std::sys::backtrace::__rust_end_short_backtrace::h7d7e47ef99abf6aa  
   7:     0x5b51de6a7c9c - rust_begin_unwind                                                   
   8:     0x5b51de03fedf - core::panicking::panic_fmt::hf8ffc7c15bfb58a0                       
   9:     0x5b51de45c4e1 - mina_indexer::ledger::account::Account::payment::h177b06f13ab13075  
  10:     0x5b51de45ef72 - mina_indexer::ledger::account::Account::apply_account_diff::habeeacd59917c716
  11:     0x5b51de1e2da7 - mina_indexer::ledger::Ledger::_apply_account_diff::h070c7ce5a8b814fd                                                                                               
  12:     0x5b51ddd6ee73 - mina_indexer::server::IndexerConfiguration::initialize::{{closure}}::h18a91e59b79d441c.4219

Do you expect this underflow to occur? Can you point to what should change in the code to ensure that it does not?