fix(rpc): use the resolved block height for eth_getBlockReceipts results

[kriss39]

Description

GetBlockReceipts resolved the requested block to a concrete height for the block lookup but then called BlockResults with blockNum.CmtHeight(), which is nil for the latest tag and makes CometBFT return the results of its current head. If a block was committed in between, the txs of block H were paired with the results of block H+1: EthMsgsFromCometBlock either panics with an index out of range (surfacing to the client as method handler crashed) or the receipts silently carry the status/gasUsed/logs of the wrong block. The resBlock == nil branch also dereferenced *blockNum.CmtHeight(), nil for latest.

This PR fetches the results of resBlock.Block.Height, like every other block query in the backend already does, and guards against a nil block.

Review notes:

• rpc/backend/blocks.go: the two-line change in GetBlockReceipts.
• tests/integration/rpc/backend/test_block_receipts.go: TestGetBlockReceiptsLatest registers Block(&10) with one indexed eth tx and a BlockResults(nil) mock that returns an empty newer block; on main it panics with index out of range [0] with length 0, with the fix it returns the receipt of block 10 and asserts that BlockResults is never called with a nil height.

Closes: #1280

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[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.68%. Comparing base (6cc4bde) to head (47105b3).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
rpc/backend/blocks.go	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1286      +/-   ##
==========================================
+ Coverage   67.58%   67.68%   +0.09%     
==========================================
  Files         321      321              
  Lines       23467    23467              
==========================================
+ Hits        15861    15884      +23     
+ Misses       6426     6396      -30     
- Partials     1180     1187       +7     

Files with missing lines	Coverage Δ
rpc/backend/blocks.go	73.22% <33.33%> (+13.38%)	⬆️

... and 2 files with indirect coverage changes

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