* fix: only one melt_quote with shared checking_id is allowed to be in a pending state.
fix mypy
add comprehensive tests
remove SQL unique index
remove test db constraint
fix lock statement
remove `test_regtest_pay_mpp_incomplete_payment`
format
* remove `test_set_melt_quote_pending_with_outputs`
* client self-rug mitigation
* fix
* format
* DB level check: error if payment reference paid or pending
* fix test
* comments
* restore
* restore
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Co-authored-by: callebtc <93376500+callebtc@users.noreply.github.com>