Fix --sort-reexports crash with non-seekable streams (e.g. stdin)

[Abdu-Ahmed]

Fixes #2393

Problem

When running isort --sort-reexports - with input piped from stdin,
isort crashed with io.UnsupportedOperation: underlying stream is not seekable. This happened because core.process() calls
output_stream.seek() as part of reexport sorting, but sys.stdout
is not seekable.

Fix

In api.sort_stream(), before calling core.process(), check whether
the output stream is seekable when sort_reexports is enabled. If not,
use an internal StringIO buffer, the same pattern already used for
non-readable streams in the atomic code path.

Test

Added regression test in test_regressions.py that verifies:

1. No exception is raised when output stream is non-seekable (sys.stdout)
2. Sorting still produces correct output when stream is seekable

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.16%. Comparing base (7317ad1) to head (aaf392c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2547   +/-   ##
=======================================
  Coverage   99.15%   99.16%           
=======================================
  Files          41       41           
  Lines        3094     3096    +2     
  Branches      669      670    +1     
=======================================
+ Hits         3068     3070    +2     
  Misses         14       14           
  Partials       12       12           

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