fix: correct error message for stdout pipe check in run_process

[nightcityblade]

Fixes #3409

The error message for the stdout=subprocess.PIPE check in run_process incorrectly said "stdin=subprocess.PIPE". This fixes it to say "stdout=subprocess.PIPE".

This is a recreation of #3398 with the same fix plus a newsfragment.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (3dd35d7) to head (6a2f6c8).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3416   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             128          128           
  Lines           19424        19417    -7     
  Branches         1318         1318           
===============================================
- Hits            19424        19417    -7     

Files with missing lines	Coverage Δ
src/trio/_subprocess.py	100.00000% <ø> (ø)
src/trio/_tests/test_subprocess.py	100.00000% <100.00000%> (ø)

... and 2 files with indirect coverage changes

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[CoolCat467]

You forgot to update the tests mate

[nightcityblade]

Good catch @CoolCat467! Updated the test to match the corrected error message — the stdin pipe check now expects stdin=subprocess.PIPE instead of stdout=subprocess.PIPE. Thanks for the review!