lending: Add overflow check in proptest (#1199)

This issue was discovered during CI:
https://github.com/solana-labs/solana-program-library/runs/1833196857

The approach is to do the same calc as `_liquidate_obligation` and make
sure that the `MockConverter` doesn't overflow `Decimal`.  This seemed
like a simpler approach than doing a complex local proptest constraint.

On top of that, it includes the proptest regression to run it on every
`cargo test`.

.gitignore

@@ -7,5 +7,4 @@ node_modules
 ./package-lock.json
 hfuzz_target
 hfuzz_workspace
-proptest-regressions
 **/*.so

token-lending/program/proptest-regressions/state/reserve.txt

@@ -0,0 +1 @@
+cc 39c6d681e691839313426bb0801cf3f46d0befb69b568a28cf6d8e752d74526b

token-lending/program/src/state/reserve.rs

@@ -854,6 +854,16 @@ mod test {
                 ..Obligation::default()
             };
 
+            // Ensure that the token conversion fits in a Decimal
+            {
+                let token_converter = MockConverter(token_conversion_rate);
+                let decimal_repay_amount = Decimal::from(obligation.max_liquidation_amount()?);
+                // Calculate the amount of collateral that will be received
+                let receive_liquidity_amount_result =
+                    token_converter.convert(decimal_repay_amount, &Pubkey::default());
+                prop_assume!(receive_liquidity_amount_result.is_ok());
+            }
+
             // Liquidate with max amount to ensure obligation can be liquidated
             let liquidate_result = Reserve::_liquidate_obligation(
                 &obligation,