Fix missing JUnit XML output for setup errors

[cty-ut]

With junit_logging=all and junit_log_passing_tests=false, a fixture setup error followed by a successful teardown produces an <error> in JUnit XML but loses the captured stdout, stderr, and logging output. The output is still present in the teardown report; write_captured_output() suppresses it because that individual phase passed.

Keep captured output when the testcase already contains an error. Output is still written at teardown, so cleanup diagnostics are included without writing the setup output twice. The existing call-failure handling is unchanged. This covers a setup-error case left outside the call-phase fix in #6274 and reproduces without third-party plugins or interlaced reports.

The regression test covers both XML families, both values of junit_log_passing_tests, and successful or failing cleanup. It checks that setup and teardown output from all three channels appears exactly once. Before the fix, the two setup-error/passing-teardown cases with logging suppression failed; the other six controls passed.

Validation:

• tox -e linting,py313: both environments passed. Python 3.13: 4701 passed, 51 skipped, 15 xfailed, 5 xpassed, 2 warnings.

• python -m pytest testing/test_junitxml.py -q: 159 passed on Python 3.12.

• Independent setup-error reproduction tests: 4 passed after the fix.

• git diff --check: passed.

• Include regression tests.

• Add the contributor to AUTHORS.

• Allow maintainers to push and squash commits.

• Add the changelog fragment changelog/15196.bugfix.rst.

• Credit AI assistance in the commit's Co-authored-by trailer.

Prepared with Codex assistance, including investigation, implementation, and test execution. The commit credits the assistance with a Co-authored-by trailer.