fix(evmrpc): return null for out-of-range tx index and fix deferred error capture

[amir-deris]

Problem

Two bugs in evmrpc/tx.go:

1. Deferred error capture was always nil — defer recordMetricsWithError(...) was called directly rather than inside a closure, so returnErr was captured at defer-registration time (always nil) instead of at function-return time. This caused metrics to never record actual errors.

2. uint32 overflow returned an error instead of null — eth_getTransactionByBlockNumberAndIndex and eth_getTransactionByBlockHashAndIndex returned an RPC error when the transaction index exceeded math.MaxUint32. The Ethereum JSON-RPC spec requires these methods to return null for an out-of-range index, not an error.

Fix

• Wrapped all defer recordMetricsWithError(...) calls in closures so returnErr is evaluated at return time.
• Introduced a typed txUint32OverflowError so the overflow case can be distinguished from other errors.
• After recording metrics, suppresses the overflow error (sets returnErr = nil) in GetTransactionByBlockNumberAndIndex and GetTransactionByBlockHashAndIndex to comply with the Ethereum JSON-RPC spec.

Test plan

• eth_getTransactionByBlockNumberAndIndex with a tx index > 0xFFFFFFFF returns null, not an error
• eth_getTransactionByBlockHashAndIndex with a tx index > 0xFFFFFFFF returns null, not an error

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	May 1, 2026, 5:40 PM

[amir-deris]

The previous format defer functionCall(...) captures arguments at the call site which results in returnErr be nil for recordMetricsWithError.

[masih]

A go convention that's worth following is to use _ prefix for named return variables to indicate that "this variable must never be directly set". I would stick with simple names too. e.g. _err.

[codecov]

Codecov Report

❌ Patch coverage is 95.65217% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.17%. Comparing base (e68b82f) to head (c1e155f).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
evmrpc/tx.go	95.65%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #3344    +/-   ##
========================================
  Coverage   59.16%   59.17%            
========================================
  Files        2097     2097            
  Lines      172613   172377   -236     
========================================
- Hits       102129   102002   -127     
+ Misses      61617    61533    -84     
+ Partials     8867     8842    -25     

Flag	Coverage Δ
sei-chain-pr	64.77% <95.65%> (?)
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/tx.go	85.48% <95.65%> (+0.46%)	⬆️

... and 52 files with indirect coverage changes

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