Handle invalid response JSON with SDK diagnostics

[tim-kos]

HTML proxy errors currently expose a parser exception when callers access a response body. Valid JSON is also reparsed on every access.

Memoize parsed bodies and invalidate them on response replacement. Wrap JSON parse failures in Transloadit::Exception::InvalidResponse, exposing HTTP status, raw body, and parser cause while keeping the message free of response contents. Explicitly reject empty/whitespace bodies because some MultiJson adapters accept them as nil. Preserve valid array, null, and false bodies.

Validation: full Ruby 3.3 suite passes (131 tests, 268 assertions; optional live-upload and signature-parity tests skipped), plus bundle exec standardrb and git diff --check. Tests cover single parsing, cache invalidation, HTML and empty responses, diagnostic access, parser causes, and existing non-object JSON behavior.

Compatibility: constrain JSON to versions below 3 because Ruby 3.1 resolves MultiJson 1.19, whose positional parsing options are incompatible with JSON 3. This dependency constraint is shared across the four independent fixes.

The four fixes also merge together cleanly. Their combined Ruby 3.3 suite passes with Node signature parity enabled: 147 tests, 321 assertions, with only the optional live upload skipped.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.42%. Comparing base (79535b2) to head (543e676).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #87      +/-   ##
==========================================
+ Coverage   99.40%   99.42%   +0.02%     
==========================================
  Files           9        9              
  Lines         336      348      +12     
  Branches       38       41       +3     
==========================================
+ Hits          334      346      +12     
  Misses          1        1              
  Partials        1        1              

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