fix(compress): do not compress 206 Partial Content responses

[dr4gan0x]

This addresses the second of the three problems reported in #5010 (compress encodes 206 Partial Content).

The middleware bails out for already-encoded, chunked, HEAD, small, non-compressible and no-transform responses, but it never checks the status code. A 206 ends up gzipped while its Content-Range header still refers to the uncompressed byte range, and the Content-Length that matched the partial body gets dropped. A range client stitching parts together from such responses gets corrupted data. This is easy to hit with a handler that does its own range handling, or when proxying an upstream that serves ranges, since the proxy helper passes the upstream status through.

The fix skips compression when the status is 206.

The new test serves a 1024 byte text/plain part with Content-Range and asserts it comes back uncompressed with status, Content-Length and Content-Range intact. It fails without the guard.

The author should do the following, if applicable

• Add tests
• Run tests
• bun run format:fix && bun run lint:fix to format the code
• Add TSDoc/JSDoc to document the code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.01%. Comparing base (e50df01) to head (a148c1d).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5020   +/-   ##
=======================================
  Coverage   79.01%   79.01%           
=======================================
  Files         154      154           
  Lines       10578    10579    +1     
  Branches     2215     2216    +1     
=======================================
+ Hits         8358     8359    +1     
  Misses       2220     2220           

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