[node-bridge] Ensure content-type is always set with multi-payloads

[ijjk]

Related Issues

This ensures a content-type is always set inside of each part of the multi-part payload as it's needed for proper parsing. This also ensures non-200 status codes/headers are passed back separate when they differ so that they can be handled independently without assuming they all match.

x-ref: https://vercel.slack.com/archives/C03AYHB6MA9
x-ref: #7507

📋 Checklist

Tests

• The code changed/added as part of this PR has been covered with tests
• All tests pass locally with yarn test-unit

Code Review

• This PR has a concise title and thorough description useful to a reviewer
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[codecov]

Codecov Report

Merging #7681 (a008e18) into main (9225246) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #7681   +/-   ##
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  Coverage   48.80%   48.80%           
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  Files         133      133           
  Lines        5331     5331           
  Branches     1338     1338           
=======================================
  Hits         2602     2602           
  Misses       2716     2716           
  Partials       13       13           

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