Fix gzip corruption in EVM JSON-RPC responses via seiLegacyHTTPGate

[masih]

The seiLegacyHTTPGate middleware wraps outside the gzip handler, so its
httptest.Recorder captures gzip-compressed bytes from the inner handler.
This causes two problems:

1. For single and all-forwarded batches the gate replays compressed
   bytes to the real http.ResponseWriter, breaking gzip framing for
   clients like ethers.js v6 (Z_DATA_ERROR).

2. For mixed/gated batches the gate calls json.Unmarshal on compressed
   bytes, which fails, causing all results to be replaced with -32603
   errors — and then writes plain JSON with a Content-Encoding: gzip
   header copied from the recorder.

Fix both paths:

• Non-gated methods (eth_, web3_, net_*) now bypass the recorder
  entirely via serveInnerWithBody, so the gzip handler writes directly
  to the real response writer.

• Recorder paths (gated sei_* singles, mixed batches) strip
  Accept-Encoding from the cloned request so the inner gzip handler
  is a no-op. The recorder captures plain JSON, merge logic works,
  and no stale Content-Encoding: gzip header is propagated.

Update two existing batch tests that relied on the recorder/merge path
for all-eth batches: add a blocked sei_* element to each so they still
exercise mergeSeiLegacyHTTPBatch through the recorder path.

[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	Apr 3, 2026, 3:59 PM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.70%. Comparing base (fcc15a3) to head (b4fef9c).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3181   +/-   ##
=======================================
  Coverage   58.69%   58.70%           
=======================================
  Files        2055     2055           
  Lines      168384   168381    -3     
=======================================
+ Hits        98838    98840    +2     
+ Misses      60742    60732   -10     
- Partials     8804     8809    +5     

Flag	Coverage Δ
sei-chain-pr	66.89% <100.00%> (?)
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/sei_legacy_http.go	72.52% <100.00%> (+3.20%)	⬆️

... and 44 files with indirect coverage changes

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[seidroid]

Successfully created backport PR for release/v6.4:

• Backport release/v6.4: Fix gzip corruption in EVM JSON-RPC responses via seiLegacyHTTPGate #3188