fix(streams): Cancel internal stream pumps before releasing their source

[sohpeach]

Merging from upstream.

This prevents pumpTo() from resuming with a dangling ReadableStreamSource pointer after forceCancel().

[ask-bonk]

LGTM

github run

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.74576% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 35.06%. Comparing base (99e1257) to head (3354a8d).

Files with missing lines	Patch %	Lines
src/workerd/api/streams/internal-test.c++	80.43%	0 Missing and 9 partials ⚠️

Additional details and impacted files

@@                       Coverage Diff                       @@
##           swallace/patch-wrappable-shims    #7165   +/-   ##
===============================================================
  Coverage                           35.05%   35.06%           
===============================================================
  Files                                 811      811           
  Lines                              258829   258885   +56     
  Branches                            21616    21625    +9     
===============================================================
+ Hits                                90735    90777   +42     
  Misses                             154975   154975           
- Partials                            13119    13133   +14     

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