feat(evmonly): add eth_getBalance RPC

[codchen]

Describe your changes and provide context

• expose eth_getBalance from the EVM-only JSON-RPC server
• read balances from the current committed EVM state
• support latest, safe, finalized, and pending, while rejecting block heights and hashes until historical state is wired
• add unit, JSON-RPC registration, and Docker integration coverage
• document the endpoint and its supported block selectors in the Autobahn README

Testing performed to validate your change

• go test -race -count=1 ./giga/evmonly/rpc/...
• go test -count=1 ./sei-tendermint/internal/rpc/core/...
• make autobahn-evmonly-integration-test (four local Docker validators, 4,000 finalized transfers, post-transfer balance checks on every validator)
• golangci-lint run ./giga/evmonly/rpc/... ./sei-tendermint/internal/rpc/core/...
• golangci-lint fmt --diff

[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	Sep 11, 2026, 5:02 AM

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.62%. Comparing base (9407b6d) to head (448897f).

Files with missing lines	Patch %	Lines
giga/evmonly/rpc/server.go	0.00%	1 Missing and 1 partial ⚠️
sei-tendermint/internal/rpc/core/mempool.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4140      +/-   ##
==========================================
- Coverage   61.58%   60.62%   -0.97%     
==========================================
  Files        2196     2080     -116     
  Lines      193825   180359   -13466     
==========================================
- Hits       119365   109336   -10029     
+ Misses      62888    60605    -2283     
+ Partials    11572    10418    -1154     

Flag	Coverage Δ
sei-chain-pr	37.18% <77.77%> (?)
sei-db	69.80% <ø> (ø)
sei-db-state-db	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
giga/evmonly/rpc/balance.go	100.00% <100.00%> (ø)
giga/evmonly/rpc/server.go	31.25% <0.00%> (-1.01%)	⬇️
sei-tendermint/internal/rpc/core/mempool.go	45.02% <0.00%> (-0.48%)	⬇️

... and 117 files with indirect coverage changes

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