Fix: wrong denom check at RegisterCoin

[zsystm]

Description

Problem:

• coinMetadata.Base is used as the denom for the token pair.
• However, the check is incorrectly performed using coinMetadata.Name, so it fails to detect if the denom is already registered.

Solution:

• Check using coinMetadata.Base, not .Name.

Proof of Work:

• Test Case
  • Running the test case with the code before the fix will fail, indicating that multiple token pairs for the same denom are created.
  • The test case passes with the changes in this PR.

Test Code Refactoring:

• I modified the test code to better match real cases. Previously, the test code incorrectly used the same value for the Base and Name fields in banktypes.BaseMetadata, which never happens in reality.
• This mistake prevented the test from catching the implementation error in the denom already registered check. By using different values for Base and Name, as in real scenarios, the implementation error would have been detected.

More Context:

• Unfortunately, there is already one case on the mainnet where multiple token pairs exist for the same denom (ibc/13B6057538B93225F6EBACCB64574C49B2C1568C5AE6CCFE0A039D7DAC02BF29).
• However, this is not a major issue because only the latest token pair registered in the denom index is actually used.
• With this patch, such cases will no longer occur.

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[poorphd]

LGTM!

[dudong2]

lgtm

[tkkwon1998]

lgtm!