evmrpc: derive block trace, sei_getCosmosTx, header gasLimit from populated data sources (CON-296)

[wen-coding]

Summary

Three eth-side block-scoped RPC paths leaned on /block_results data that isn't populated under Autobahn — they returned empty (or wrong) results regardless of block contents.

• Block trace backend (debug_traceBlockBy*, sei_traceBlockBy*ExcludeTraceFail) now iterates the block's own tx list.
• sei_getCosmosTx looks up the cosmos hash directly via receipt.TransactionIndex, no iteration needed.
• getHeader (trace path's eth-block construction) sources gasLimit from sdkCtx.ConsensusParams(), matching the block.go:489-491 pattern already used by eth_getBlockByNumber. Under legacy this also fixes a latent silent fallback to DefaultBlockGasLimit.

Existing .iox fixtures only asserted response kind, so empty results / early-exit error paths looked indistinguishable from the fix; the trace fixtures now bind to the seeded deploy block and assert non-empty output, and a positive-case getCosmosTx-deploy.iox was added.

Test plan

• go test ./evmrpc/ (22.3s)
• go test ./evmrpc/tests/ (27.7s)
• Full RPC IO suite, docker localnet, Autobahn cluster: 161/161
• Full RPC IO suite, docker localnet, legacy cluster: 160/160 (run on the earlier 5-fixture state; new fixture additive)
• Buggy binary (block-trace fix reverted) on Autobahn: 156/160 — exactly the four updated trace fixtures fail
• Buggy binary (sei_getCosmosTx fix reverted) on Autobahn: 160/161 — exactly the new getCosmosTx-deploy.iox fixture fails
• gofmt -s -l clean

[cursor]

PR Summary

Medium Risk
Changes core RPC behaviors for debug_traceBlockBy*/sei_traceBlockBy* and sei_getCosmosTx, which could affect downstream clients if any edge cases around tx ordering/indexing or consensus param availability are mishandled.

Overview
Fixes block-scoped trace RPCs to iterate over transactions from the resolved block (tmBlock.Block.Txs) instead of /block_results tx results, which may be unpopulated in some environments and previously produced empty traces.

Updates sei_getCosmosTx to return the Cosmos tx hash directly from the EVM receipt’s BlockNumber + TransactionIndex (with bounds checking) rather than scanning decoded transactions, and changes header gas-limit derivation to use SDK ConsensusParams at the block height with a default fallback.

Tightens tests: unit tests now cover consensus-params gas limit behavior without relying on BlockResults, and RPC IO fixtures bind block hash/number from a seeded deploy receipt and assert non-empty trace output; adds a positive-case sei_getCosmosTx fixture.

^{Reviewed by Cursor Bugbot for commit f33e827. Bugbot is set up for automated code reviews on this repo. Configure here.}

[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	May 16, 2026, 9:42 PM

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.29%. Comparing base (823a78d) to head (f33e827).

Files with missing lines	Patch %	Lines
evmrpc/association.go	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3449      +/-   ##
==========================================
- Coverage   59.29%   59.29%   -0.01%     
==========================================
  Files        2125     2125              
  Lines      175629   175607      -22     
==========================================
- Hits       104144   104129      -15     
+ Misses      62404    62400       -4     
+ Partials     9081     9078       -3     

Flag	Coverage Δ
sei-chain-pr	67.33% <75.00%> (?)
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
evmrpc/simulate.go	74.34% <100.00%> (+0.15%)	⬆️
evmrpc/association.go	40.19% <50.00%> (-4.35%)	⬇️

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