Harden EVM field-bloat validation

[Kbhat1]

Summary

• reject raw Cosmos Tx.Signatures on EVM txs instead of trusting GetSignaturesV2(), which can report zero signatures when SignerInfos is empty
• reject protobuf-sized bloat in MsgEVMTransaction and embedded ethTx payloads by checking canonical serialized sizes, covering external Derived bytes and extra inner tx fields
• add focused regression tests for the ante decorator, EvmStatelessChecks, the main EVM message type, and the mirrored Giga message validation path

Test plan

• Unit tests

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Mar 13, 2026, 7:59 PM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.41%. Comparing base (bb2c5b3) to head (e202655).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3073      +/-   ##
==========================================
- Coverage   58.41%   58.41%   -0.01%     
==========================================
  Files        2081     2110      +29     
  Lines      171790   174428    +2638     
==========================================
+ Hits       100352   101887    +1535     
- Misses      62504    63529    +1025     
- Partials     8934     9012      +78     

Flag	Coverage Δ
sei-db	70.41% <ø> (-0.22%)	⬇️

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Files with missing lines	Coverage Δ
app/ante/evm_checktx.go	35.88% <ø> (+2.12%)	⬆️
giga/deps/xevm/types/message_evm_transaction.go	41.50% <ø> (ø)
x/evm/ante/no_cosmos_fields.go	100.00% <ø> (+5.55%)	⬆️
x/evm/types/message_evm_transaction.go	77.35% <ø> (ø)

... and 336 files with indirect coverage changes

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