Fix flaky TestRPC JSON-RPC test due to hardcoded port

[masih]

Make the port and unix socket unique per JSON-RPC test.

Flaked on main.

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Feb 22, 2026, 4:34 AM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.83%. Comparing base (f4b318c) to head (439fd70).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2945      +/-   ##
==========================================
+ Coverage   57.82%   57.83%   +0.01%     
==========================================
  Files        2110     2110              
  Lines      174122   174057      -65     
==========================================
- Hits       100681   100663      -18     
+ Misses      64464    64427      -37     
+ Partials     8977     8967      -10     

Flag	Coverage Δ
sei-db	68.49% <ø> (+0.06%)	⬆️

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see 26 files with indirect coverage changes

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