fix: prevent pipeline body replay on redirect

[mcollina]

This relates to...

Fixes #5204.

Rationale

undici.pipeline() uses a live writable-side request body that cannot be replayed safely. Redirect handling was only blocking redirects after the body had been consumed, which left a race where a redirect could be followed before any bytes were read.

Changes

• mark pipeline request bodies as non-replayable from the start
• add a regression test that forces a redirect before the pipeline body is consumed
• wait for tspl completion in the redirect pipeline tests

Features

N/A

Bug Fixes

• prevent pipeline request bodies from being replayed across redirects
• make test/redirect-pipeline.js deterministic

Breaking Changes and Deprecations

None.

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.28%. Comparing base (397bc13) to head (346b71c).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5274   +/-   ##
=======================================
  Coverage   93.27%   93.28%           
=======================================
  Files         110      110           
  Lines       36380    36388    +8     
=======================================
+ Hits        33934    33943    +9     
+ Misses       2446     2445    -1     

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[trivikr]

Needs rebase to include node 26 FFI build from https://www,github.com/nodejs/undici/pull/5275