Raise a clear error when xdist is enabled after pytest-cov has started

[pctablet505]

Summary

Fixes #740.

When another plugin only enables pytest-xdist after pytest-cov's own
pytest_load_initial_conftests has already run (for example, a plugin that
adds -n/--dist based on addopts read from a config file), pytest-cov
has already committed to centralised (Central) coverage measurement by
the time xdist calls pytest_configure_node. That produced a confusing
AttributeError: 'Central' object has no attribute 'configure_node'.

Coverage measurement for the master process has to start very early (during
pytest_load_initial_conftests) so that code imported while conftests load
is still measured, so we can't reliably know in advance whether xdist will
be turned on later by some other plugin. Rather than silently swapping
engines mid-run (which would mean discarding coverage already collected),
this detects the mismatch when it happens and raises the existing
DistCovError with a message that explains what happened and how to avoid
it.

Added a regression test that simulates a plugin enabling xdist late, and
confirmed it reproduces the original AttributeError without the fix and
raises a clean DistCovError with it.

Test plan

• Added test_xdist_enabled_after_initial_conftests in tests/test_pytest_cov.py
• Verified the new test fails with the original AttributeError on master
• Ran the full test suite (pytest tests/test_pytest_cov.py); all tests pass except
  test_celery, which fails identically on master due to a missing testcontainers/Docker
  dependency in my environment, unrelated to this change
• ruff check and ruff format --check pass on the touched files