Preserve '.[' in long report headlines (getmodpath sanitization)

[rowan-stein]

Summary

• Parametrized test ids containing the substring ".[" are incorrectly mutated in long report output/headlines: any ".[" becomes "[". Example: "..[" becomes ".[" in the failure header. This also affects tools relying on headlines (e.g., VS Code test discovery heuristics).

Observed failure

bug.py F                                                                 [100%]

=================================== FAILURES ===================================
_________________________________ test_boo[.[] _________________________________

a = '..['

    @pytest.mark.parametrize("a",["..["])
    def test_boo(a):
>       assert 0
E       assert 0

bug.py:6: AssertionError
============================== 1 failed in 0.06s ===============================

The "test_boo[..[]" is rendered as "test_boo[.[]" in the failure headline.

Root cause

• src/_pytest/python.py: in PyobjMixin.getmodpath(), the result is returned as s.replace(".[", "[").
• This was historically intended to avoid dotted paths like test_gen.[0] from yield-based tests (removed in pytest 4.0).
• With yield tests gone, this sanitization is obsolete and now corrupts parameter ids which legitimately contain the sequence ".[".

Proposed fix

• Change getmodpath() to return s unchanged, and add a brief comment noting yield tests removal.

Reproduction steps

1. Create bug.py:

import pytest

@pytest.mark.parametrize("a", ["..["])
def test_boo(a):
    assert False

2. Run: pytest -vv bug.py.
3. Observe the failure headline shows test_boo[.[] instead of test_boo[..[].

Acceptance criteria

• Long report failure headlines preserve the exact parameter id text; no mutation of ".[" sequences inside ids.
• Nodeids remain unchanged.
• Add a regression test using pytester that asserts test_boo[a..[b]] appears unchanged in -vv headline output.

Notes

• Yield tests have been removed since pytest 4.0; no remaining path should produce node segments like [0] that would justify the sanitization.
• No CI run is necessary for this PR per project rules.