[PR #14443/a481f264 backport][9.0.x] Fix strict options from addopts

[patchback]

This is a backport of PR #14443 as merged into main (a481f26).

Closes #14442

This fixes a pytest 9 regression where --strict-markers and
--strict-config were ignored when configured through addopts

addopts is parsed after the initial config discovery step, so the
OverrideIniAction compatibility path updated the parsed argparse namespace,
but the generated ini overrides were not applied back into _inicfg

As a result, later getini() calls still saw the default values for
strict_markers and strict_config

This change applies parsed ini overrides after parsing the full argument list
including addopts, and clears the ini cache so later getini() calls use the
updated values

Added regression coverage for:

• --strict-config from addopts
• --strict-markers from addopts

Tested with:

python -m pytest -q \
  testing/test_config.py::TestParseIni::test_strict_config_from_addopts \
  testing/test_mark.py::test_strict_markers_from_addopts

python -m pytest -q \
  testing/test_config.py::TestParseIni::test_invalid_config_options \
  testing/test_mark.py::test_strict_prohibits_unregistered_markers