Add ordered parameter to RaisesGroup

[anneheartrecord]

Summary

Closes #14580.

pytest.RaisesGroup currently ignores the order of the exceptions in a group and pairs them up with a greedy matching algorithm. This PR adds an opt-in ordered: bool = False parameter. When ordered=True, the expected exceptions are matched against the raised exceptions positionally (expected[i] vs actual[i]), which:

• lets you assert that exceptions were raised in a particular order (the feature request in Support checking exception order in RaisesGroup #14580), and
• side-steps the documented greedy-algorithm pitfall, where a valid pairing exists but the greedy algorithm fails to find it.

The default (ordered=False) preserves the existing order-insensitive behavior, so this is fully backwards compatible.

Example

# order is asserted
with pytest.RaisesGroup(ValueError, TypeError, ordered=True):
    raise ExceptionGroup("msg", [ValueError("foo"), TypeError("bar")])

# wrong order now fails
with pytest.RaisesGroup(ValueError, TypeError, ordered=True):
    raise ExceptionGroup("msg", [TypeError("bar"), ValueError("foo")])
# -> Raised exception group did not match: At index 0: `TypeError()` is not an instance of `ValueError`

Implementation notes

• Added ordered to all RaisesGroup.__init__ overloads and the runtime signature, stored as self.ordered.
• New focused helper RaisesGroup._check_exceptions_ordered does the positional matching and produces failure messages that point at the offending index (At index N: ...) and report length mismatches.
• RaisesGroup._check_exceptions short-circuits to the ordered path when ordered=True; the greedy path is untouched.
• __repr__ shows ordered=True only when set (parallel to flatten_subgroups).
• Docstring, doc/en/how-to/assert.rst, and a changelog/14580.feature.rst newsfragment updated; added myself to AUTHORS.

Testing

• testing/python/raises_group.py: new test_ordered (success in/out of order, wrong order, wrong length, nested groups + RaisesExc, greedy-pitfall case) and test_ordered_repr.
• testing/typing_raises_group.py: ordered= accepted alongside the other parameters.
• uv run pytest testing/python/raises_group.py testing/python/raises.py → 67 passed; with --doctest-modules src/_pytest/raises.py → 68 passed.
• ruff check / ruff format --check clean on all touched files.