fix(eth): use 0-indexed transactionPosition in trace_block

[rvagg]

Thanks to Yan from Blockscout for picking this one up. It's been present since the beginning for these APIs. Confirmed incorrect by querying public Ethereum APIs to make sure they are all zero-based.

curl "https://filecoin.chain.love/rpc/v1" -s -X POST -H "Content-Type: application/json" \
  --data '{"id":1,"jsonrpc":"2.0","method":"trace_block","params":["latest"]}' \
  | jq '.result | map(.transactionPosition) | unique' | head -5
[
  1,
  2,
  3,
  4,

$ curl "https://docs-demo.quiknode.pro/" -s -X POST -H "Content-Type: application/json" \
  --data '{"id":1,"jsonrpc":"2.0","method":"trace_block","params":["latest"]}' \
  | jq '.result | map(.transactionPosition) | unique' | head -5
[
  0,
  1,
  2,
  3,

cc @LesnyRumcajs

[Copilot]

Pull request overview

This PR fixes the Ethereum trace API’s transactionPosition indexing so trace responses align with Ethereum’s zero-based transaction ordering and match Lotus’s block transaction indices.

Changes:

• Move EthTraceBlock’s message index increment so the first non-system transaction is reported as position 0 instead of 1.
• Update integration-test expectations in TestTraceFilter to reflect zero-based transactionPosition.
• Document the API behavior change in the unreleased changelog.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
node/impl/eth/trace.go	Adjusts transactionPosition assignment for block-derived trace results.
itests/eth_transactions_test.go	Updates trace filter assertions for the new zero-based positions.
CHANGELOG.md	Records the trace API off-by-one fix for the next release.

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