evmrpc: derive block trace, sei_getCosmosTx, header gasLimit from populated data sources (CON-256)

[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.

Supersedes #3449 (closed when the head branch was renamed).

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
Medium risk because it changes core RPC behaviors for block tracing and sei_getCosmosTx, and alters how Ethereum headers derive GasLimit, which could affect downstream clients if edge cases (missing consensus params, tx index mismatches) are mishandled.

Overview
Fixes several EVM RPC paths that previously depended on Tendermint /block_results data (which may be missing under Autobahn), causing empty/incorrect responses.

debug_traceBlockBy* and sei_traceBlockBy*ExcludeTraceFail now iterate over the block’s own Txs list, sei_getCosmosTx now maps EVM→Cosmos tx hash directly via the receipt’s TransactionIndex, and getHeader now sources GasLimit from sdkCtx.ConsensusParams() (falling back to DefaultBlockGasLimit only when consensus params are absent).

Tests/fixtures are updated to assert non-empty trace output by binding to the seeded deploy tx’s block, and a new positive RPC-IO fixture is added for sei_getCosmosTx.

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

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
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[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, 10:28 PM