Tidy app gas accounting loop and tests

[masih]

• Remove nonzeroTxsCnt: declared and incremented but never read.
• Remove a stale "Skipping" inline comment that no longer describes
  the branch it sits next to.
• Reduce allocations and optimise for fast path on decode failure.

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	May 19, 2026, 2:24 PM

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.30%. Comparing base (1b89b44) to head (358b09a).

Files with missing lines	Patch %	Lines
app/app.go	87.50%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3464      +/-   ##
==========================================
- Coverage   59.30%   59.30%   -0.01%     
==========================================
  Files        2127     2127              
  Lines      175877   175868       -9     
==========================================
- Hits       104301   104293       -8     
+ Misses      62489    62488       -1     
  Partials     9087     9087              

Flag	Coverage Δ
sei-chain-pr	50.28% <87.50%> (?)
sei-db	70.62% <ø> (+0.21%)	⬆️

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

Files with missing lines	Coverage Δ
app/app.go	69.23% <87.50%> (-0.17%)	⬇️

... and 1 file with indirect coverage changes

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[cursor]

PR Summary

Medium Risk
Touches proposal-time block gas accounting. Changes are small and covered by updated/new unit tests, but the code sits on the consensus-critical proposal acceptance path.

Overview
Proposal-time block gas validation is tightened and simplified. checkTotalBlockGas now rejects proposals immediately, adds explicit uint64 overflow guards for lint issues, and streamlines MaxGas/MaxGasWanted checks.

Gasless heuristics/tests are cleaned up. couldBeGaslessTransaction is simplified to a single conservative message-type check, and the test suite is refactored with reusable helpers.

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