Piping suite: absorb special-buffer pipe writes (audit G9)

[jasnell]

More streams test consolidations

[ask-bonk]

src/tests/streams/piping/AGENTS.md:84 duplicates the two regression names already listed on the preceding line, leaving the security-regression inventory inaccurate.

[ask-bonk]

I'm Bonk, and I've done a quick review of your PR.

Consolidates special-buffer piping coverage into the shared streams piping suite.

1. Low: src/tests/streams/piping/AGENTS.md:84 duplicates the security regression names, leaving the suite inventory inaccurate. Posted a suggestion comment.

github run

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37.01%. Comparing base (40cc902) to head (aaaf8d4).

Additional details and impacted files

@@                           Coverage Diff                            @@
##           jasnell/streams-test-consolidation-9    #7181      +/-   ##
========================================================================
- Coverage                                 37.03%   37.01%   -0.02%     
========================================================================
  Files                                       794      793       -1     
  Lines                                    247909   247822      -87     
  Branches                                  19572    19572              
========================================================================
- Hits                                      91805    91743      -62     
+ Misses                                   145013   144998      -15     
+ Partials                                  11091    11081      -10     

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