fix: show config errors once at session start instead of per-test

[croc100]

Problem

When alembic.ini was missing, pytest-mrt produced the same error for
every collected test. A default project has 7 built-in tests, so the
output was 7 identical blocks each with a double traceback:

ERROR at setup of test_mrt_single_head
alembic.ini not found: 'alembic.ini'
...Django hint...
During handling of the above exception, another exception occurred:
alembic.ini not found: 'alembic.ini'
...same message again...

(repeated 7 times)

Fix

• Add pytest_sessionstart hook that validates MRTConfig once before
  any test runs. Fatal config errors call pytest.exit() -- one clear
  message, session stops immediately.
• Break the exception chain in the mrt fixture by capturing the error
  string before calling pytest.fail(). This eliminates the "During
  handling of the above exception" noise.
• Simplify MRTFixture.__init__ error message: the Django hint is now
  shown only via pytest_sessionstart (not duplicated in the fixture).

Before / After

Before:

ERROR at setup of test_mrt_single_head    <- x7
alembic.ini not found: 'alembic.ini'
...
During handling of the above exception, another exception occurred:
alembic.ini not found: 'alembic.ini'
...

After:

Exit: alembic.ini not found: 'alembic.ini'

Check the path and update MRTConfig(alembic_ini=...) in your conftest.py.

If you are using Django migrations (not Alembic), set django_settings instead:
    ...

See: https://croc100.github.io/pytest-mrt/quickstart/

Test plan

• All existing tests pass (431 passed, 17 skipped)
• Updated test_mrt_fixture_raises_on_missing_alembic_ini to match revised message
• Manual: pytest test_migrations.py with missing alembic.ini shows one error and exits cleanly

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

❌ Patch coverage is 81.25000% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pytest_mrt/plugin.py	53.33%	5 Missing and 2 partials ⚠️
pytest_mrt/commands/init.py	93.93%	1 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!