fix: keep local coverage central without xdist workers

[Samuel-Hills]

Summary

Fixes coverage setup when --dist is configured without any xdist execution
environments.

Previously, pytest-cov selected the distributed controller whenever --dist
was non-default, even when tests still ran in the current process. This
prevented subprocess coverage setup.

Changes

• Select the distributed controller only when xdist has workers or execution
  environments.
• Preserve existing behavior for -n and --tx runs.
• Add regression coverage for bare --dist.

Validation

• Full test suite: 204 passed, 6 skipped
• Ruff check: passed
• git diff --check: passed

Fixes #667.