fix: Fix err ack in ibc v2 middleware

[Eric-Warehime]

Description

Fixes the err ack return value in the ibc v2 middleware for erc20s.

Previously we were swallowing the return value of the keeper invocation. After this change we properly return an error ack if the app's OnRecvPacket call passes, but the middleware keeper's OnRecvPacket call fails.

Adde test covers this case.

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Author Checklist

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I have...

• tackled an existing issue or discussed with a team member
• left instructions on how to review the changes
• targeted the main branch

[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.62%. Comparing base (83a3a6c) to head (9139176).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
x/erc20/v2/ibc_middleware.go	57.14%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #967      +/-   ##
==========================================
+ Coverage   65.56%   65.62%   +0.06%     
==========================================
  Files         319      319              
  Lines       22220    22225       +5     
==========================================
+ Hits        14569    14586      +17     
+ Misses       6480     6476       -4     
+ Partials     1171     1163       -8     

Files with missing lines	Coverage Δ
x/erc20/v2/ibc_middleware.go	69.41% <57.14%> (-1.84%)	⬇️

... and 3 files with indirect coverage changes

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[srdtrk]

Lgtm. Left a hardening suggestion, I'll leave it up to you to implement it or not. I can verify that this fixes the original issue