unittest MigrateUnbondingRecords

[asalzmann]

Summary

Fix two issues in the migration logic
(1) divide by zero wasn't being caught
(2) LegacyDec shouldn't be used (it takes an int like 250 and makes it 0.00...00250)

Test plan
Add unittests

[riley-stride]

Tests look great. Verified you're covering

• all the error cases in MigrateUnbondingRecords (except the seemingly impossible "user redemption record not found" error on ln 342)
• all the record types
• zero values for at least one record type. I believe (check me) that 0 is not possible for records in status other than UNBONDING_QUEUE. If that's the case, we have full coverage of the cases where NativeTokenAmount could be 0.

Open question — I see a bunch of other uses of sdkmath.ZeroInt() across the codebase. Should those be removed, or only in the case where it's being used to check whether a Dec is 0-value?

[riley-stride]

Open question — I see a bunch of other uses of sdkmath.ZeroInt() across the codebase. Should those be removed, or only in the case where it's being used to check whether a Dec is 0-value?

Sam and I connected about this. Sounds like the only case that breaks is comparing a value as follows: X == sdkmath.ZeroInt() (it compares the pointers rather than values, so even if X is 0 this equality will return false).

I searched through the rest of the codebase and can verify we don't use this elsewhere.

Good to go on this PR!