perf: Reduce unnecessary validation on responses

[FrederikBolding]

Previously we were validating that responses were JSON, but we already do that elsewhere and are guaranteed for the responses to be valid by the time we get to isValidResponse. Additionally, we don't need an entirely byte accurate length for the response, if the Snap goes beyond the limit they simply won't receive a response from the Snap.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.03%. Comparing base (2933de0) to head (e7257ee).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3350      +/-   ##
==========================================
- Coverage   98.03%   98.03%   -0.01%     
==========================================
  Files         402      402              
  Lines       11110    11109       -1     
  Branches     1754     1754              
==========================================
- Hits        10892    10891       -1     
  Misses        218      218              

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[Mrtenz]

This won't be necessary anymore after #3322 😄