2.9.0 regression: _StringException traceback leaks into error reports for stdlib unittest.TestResult consumers

[jelmer]

testtools 2.9.0 leaks its own internal traceback into the error string seen by stdlib unittest.TestResult consumers of ExtendedToOriginalDecorator. 2.8.x reported just testtools.testresult.real._StringException; 2.9.0 reports a full traceback pointing into testtools/testresult/real.py.

import unittest
from io import BytesIO
import subunit

client = unittest.TestResult()
subunit.TestProtocolServer(client).readFrom(BytesIO(b"test x\nerror x\n"))
print(client.errors[0][1])

2.8.x:

testtools.testresult.real._StringException

2.9.0:

Traceback (most recent call last):
  File ".../testtools/testresult/real.py", line 1991, in addError
    self.decorated.addError(test, details=details)  # type: ignore[call-arg]
  File ".../unittest/result.py", line 17, in inner
    return method(self, *args, **kw)
TypeError: TestResult.addError() got an unexpected keyword argument 'details'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File ".../testtools/testresult/real.py", line 2118, in _details_to_exc_info
    raise _StringException(_details_to_str(details, special="traceback"))
testtools.testresult.real._StringException

Breaks subunit CI: https://github.com/testing-cabal/subunit/actions/runs/24862737296/job/72791864617?pr=102.