Handle :: in loadscope parametrized ids

[itscloud0]

Problem

Fixes #1335.

LoadScopeScheduling._split_scope() currently takes the last :: in the nodeid without accounting for parametrized ids. Values like ::1 or cafe:cafe::cafe make --dist=loadscope split a single module into multiple work units, so related tests can be scattered across workers and pay repeated fixture setup costs.

Solution

Track the last top-level :: outside parametrization brackets when deriving the loadscope key.

This keeps the existing class-vs-module grouping behavior, while ignoring :: that only appear inside parametrized ids. I also added:

• a parser-level regression for function and class-method nodeids
• an acceptance test showing parametrized IPv6 ids stay grouped per module under --dist=loadscope
• the required changelog fragment

Verification

• python -m pytest testing/test_loadscope.py -q
• python -m pytest testing/acceptance_test.py -q -k 'test_by_module or test_by_class or test_by_module_with_double_colons_in_parametrize_ids'
• python -m ruff check src/xdist/scheduler/loadscope.py testing/acceptance_test.py testing/test_loadscope.py

Limitations

This only changes loadscope scope parsing. loadfile and loadgroup behavior is unchanged.