fix(rpc): align eth_getBalance response with execution-apis spec

[songgaoye]

Closes: #XXX

Description

---

For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

---

For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.17%. Comparing base (f959eb9) to head (d69e7f8).
⚠️ Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #970      +/-   ##
===========================================
+ Coverage    44.15%   44.17%   +0.01%     
===========================================
  Files          196      196              
  Lines        16896    16896              
===========================================
+ Hits          7461     7463       +2     
+ Misses        8645     8644       -1     
+ Partials       790      789       -1     

Files with missing lines	Coverage Δ
rpc/backend/blocks.go	78.99% <100.00%> (+0.59%)	⬆️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.