fix(giga): order fee validation checks correctly

[arajasek]

Describe your changes and provide context

This PR improves and cleans up the validation performed by the Giga executor before running a tx:

• Moves all validation into its own method
• Bumps nonce if and only if the nonce is correct
  • Basically "if nonce is correct, it will get bumped, regardless of any other potential failures"
• Orders the various validation checks to match v2, so that in the edge-case where multiple things fail, we respect v2's behaviour.

Testing performed to validate your change

[arajasek]

Draft until tested on pacific today

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Mar 11, 2026, 1:56 PM

[codecov]

Codecov Report

❌ Patch coverage is 0% with 87 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.31%. Comparing base (2bb2537) to head (9d42f69).

Files with missing lines	Patch %	Lines
app/app.go	0.00%	87 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3017      +/-   ##
==========================================
- Coverage   58.33%   58.31%   -0.02%     
==========================================
  Files        2079     2079              
  Lines      171885   171920      +35     
==========================================
- Hits       100268   100258      -10     
- Misses      62671    62725      +54     
+ Partials     8946     8937       -9     

Flag	Coverage Δ
sei-chain-pr	48.27% <0.00%> (?)
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
app/app.go	68.07% <0.00%> (-2.53%)	⬇️

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