prevent capteesys from duplicating output when -s disables global capture

[ron-42]

Summary

Using the capteesys fixture with -s (--capture=no) printed output twice, even if the fixture wasn’t used in the test body. This issue is tracked in [pytest-dev/pytest#13784](https://github.com/pytest-dev/pytest/issues/13784) and is fixed by this PR.

Fixes: pytest-dev/pytest#13784

---

Root Cause

capteesys always instantiated a tee-ing SysCapture, so when global capture was disabled (-s), output went directly to the terminal and was also duplicated through the tee path.

---

Fix

Enable tee only when global capture is active. When global capture is disabled, capteesys yields a non-teeing fixture to avoid duplication.

Changes localized to: src/_pytest/capture.py (capteesys fixture)

---

Tests

Added regression test: test_capteesys_issue.py

• Reproduces duplication with -s and validates the fix.

---

Behavior Comparison

Before: Running pytest -s with capteesys in the test signature duplicated stdout/stderr.

After: pytest -s prints each line exactly once (no duplication). With capture enabled, capteesys continues to tee and capture as expected.

---

Backwards Compatibility

• No public API changes.
• Behavior now aligns with the selected global --capture mode.

---

Affected Files

• src/_pytest/capture.py
• test_capteesys_issue.py