reverted TxMempool.CheckTx to previous behavior (CON-266)

[pompon0]

#3334 made TxMempool.CheckTx and Application.CheckTx return an error instead of ResponseCheckTx with an error code. This PR reverts to the previous behavior. Since there is no case in which Application.CheckTx returns unwrapped error, the signature has been tightened to always return ResponseCheckTxV2. Proxy.CheckTxSafe() keeps returning (*ResponseCheckTxV2,error) though, so that in case of panic the stack trace is not send as part of the RPC response.

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	May 5, 2026, 12:14 PM

[codecov]

Codecov Report

❌ Patch coverage is 36.11111% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.11%. Comparing base (52d368f) to head (66e60d0).

Files with missing lines	Patch %	Lines
app/abci.go	0.00%	12 Missing ⚠️
sei-tendermint/internal/mempool/mempool.go	70.00%	2 Missing and 1 partial ⚠️
sei-cosmos/baseapp/abci.go	0.00%	2 Missing ⚠️
sei-tendermint/abci/types/application.go	0.00%	2 Missing ⚠️
sei-tendermint/abci/types/mocks/application.go	0.00%	2 Missing ⚠️
sei-tendermint/abci/types/types.go	75.00%	1 Missing ⚠️
sei-tendermint/test/e2e/app/app.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3386      +/-   ##
==========================================
+ Coverage   59.08%   59.11%   +0.03%     
==========================================
  Files        2100     2100              
  Lines      173008   173003       -5     
==========================================
+ Hits       102219   102279      +60     
+ Misses      61926    61868      -58     
+ Partials     8863     8856       -7     

Flag	Coverage Δ
sei-chain-pr	62.84% <36.11%> (?)
sei-db	70.41% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sei-cosmos/types/errors/abci.go	91.42% <ø> (+7.21%)	⬆️
sei-tendermint/internal/mempool/metrics.go	100.00% <100.00%> (ø)
sei-tendermint/internal/proxy/proxy.go	93.47% <100.00%> (ø)
sei-tendermint/abci/types/types.go	72.22% <75.00%> (+0.12%)	⬆️
sei-tendermint/test/e2e/app/app.go	0.00% <0.00%> (ø)
sei-cosmos/baseapp/abci.go	61.92% <0.00%> (ø)
sei-tendermint/abci/types/application.go	39.28% <0.00%> (ø)
sei-tendermint/abci/types/mocks/application.go	12.88% <0.00%> (+0.38%)	⬆️
sei-tendermint/internal/mempool/mempool.go	72.85% <70.00%> (+0.44%)	⬆️
app/abci.go	62.50% <0.00%> (-3.91%)	⬇️

... and 3 files with indirect coverage changes

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