fix: remove potential runtime panic in x/feegrant

[ulbqb]

Description

I added safe type casting and then removed potential runtime panic.

Motivation and context

Unsafe type casting may cause potential runtime panic.

How has this been tested?

Screenshots (if appropriate):

Checklist:

• I followed the contributing guidelines and code of conduct.
• I have added a relevant changelog to CHANGELOG.md
• I have added tests to cover my changes.
• I have updated the documentation accordingly.
• I have updated API documentation client/docs/swagger-ui/swagger.yaml

[codecov]

Codecov Report

Merging #720 (d3573e9) into main (5ed82db) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #720   +/-   ##
=======================================
  Coverage   61.96%   61.96%           
=======================================
  Files         877      877           
  Lines       98764    98813   +49     
=======================================
+ Hits        61200    61233   +33     
- Misses      33955    33980   +25     
+ Partials     3609     3600    -9     

Impacted Files	Coverage Δ
x/feegrant/filtered_fee.go	64.00% <100.00%> (+6.25%)	⬆️
x/token/validation.go	87.27% <0.00%> (-3.64%)	⬇️
x/collection/msgs.go	39.81% <0.00%> (-3.15%)	⬇️
x/collection/collection.go	78.57% <0.00%> (-1.79%)	⬇️
x/wasm/keeper/keeper.go	85.80% <0.00%> (-0.37%)	⬇️
x/foundation/client/testutil/query.go	100.00% <0.00%> (ø)
crypto/ledger/ledger_secp256k1.go	52.89% <0.00%> (+0.75%)	⬆️
x/token/msgs.go	34.51% <0.00%> (+2.03%)	⬆️

[0Tech]

It would be better to provide more meaningful names to the mocks.