fix: make the ESSR tunnel byte-transparent

[Sotatek-Patrick-Vu]

The ESSR tunnel round-tripped bodies through text, corrupting unusual line separators, whitespace, and binary payloads.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.43%. Comparing base (b5d8e64) to head (9ed58e4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #456      +/-   ##
==========================================
- Coverage   88.44%   88.43%   -0.01%     
==========================================
  Files          26       26              
  Lines        5938     5934       -4     
==========================================
- Hits         5252     5248       -4     
  Misses        686      686              

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