Fix max_body_size for Content-Encoding: br

[oalders]

Fixes #229.

IO::Uncompress::Brotli::decompress is a one-argument method, so passing the limit as a second argument was an XS usage error rather than a size cap. Every brotli response therefore failed to decode whenever a limit was set, regardless of size -- an 11-byte body died under a 1 GB limit.

unbro($in, $max) is the only brotli interface that accepts a size. It decodes into a buffer of that many octets, so the limit bounds the allocation directly rather than being checked after the fact. It does not commit the buffer up front: RSS stays flat decoding a small payload under a large limit.

One consequence worth knowing: unbro reports "output buffer too small" and "this is not valid brotli" identically, so the error cannot say which occurred and names both possibilities. That is unlike the gzip and bzip2 branches, which can tell the two apart.

Test

t/message-decode-brotlibomb.t asserted only that decoding died when the body exceeded the limit. It did die -- on the arity error -- so it passed for the wrong reason since it was written. It now also asserts on the error message, and checks that a body under the limit still decodes, which is the case that was actually broken.

Verified the new assertions fail against the current master and pass here:

master     exit=2   (error-message assertion fails; under-limit test dies)
this branch exit=0

Note that prove -I is not enough to check this if you have --lib in a .proverc, since ./lib then wins over -I. Run the file with perl -I<path> directly.

The plan is now done_testing() rather than a hardcoded count.

Scope

Code, test and a Changes line only. max_body_size has no Pod on master, so nothing to update here.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.50%. Comparing base (746001b) to head (8cc1217).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #230      +/-   ##
==========================================
- Coverage   94.57%   94.50%   -0.08%     
==========================================
  Files          10       10              
  Lines        1254     1256       +2     
  Branches      350      351       +1     
==========================================
+ Hits         1186     1187       +1     
  Misses         14       14              
- Partials       54       55       +1     

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