fix: avoid duplicate report rows for pytest 9 subtests

[xuanshen666]

What

When a test uses the new subtests fixture from pytest 9 and has mixed pass/fail subtest outcomes, pytest-html generates two rows for the same test (one "Passed" and one "Failed").

Why

Each subtest emits its own pytest_runtest_logreport with when="call" and the same nodeid. pytest-html stores these per-subtest reports as separate result rows instead of collapsing them into the parent test's single row.

How

In BaseReport.pytest_runtest_logreport, skip subtest reports (identified by the context attribute). The parent test's own call report already contains the full subtest details, so a single row is still produced with the correct outcome and log.

Verification

• Added test_subtests_single_row to testing/test_unit.py (skipped on pytest < 9).
• Before the fix the test produced 2 rows (Passed, Failed); after the fix it produces 1 row (Failed).
• pytest testing/test_unit.py passes (8 passed).

Closes #981

[xuanshen666]

Note: the failing pre-commit.ci check is the djlint hook on src/pytest_html/resources/index.jinja2 (H024/H056/H057/H043). This PR does not touch that file, so the failure is pre-existing on master — likely djlint version drift, since those rules are not in the [tool.djlint] ignore list. The files changed here (src/pytest_html/basereport.py, testing/test_unit.py) pass black, flake8, mypy, and reorder-python-imports.