Convert the bedtools intersect integration tests to the project's test conventions #236

tests/integration/bedtools/test_intersect.py predates the project's test conventions and none of its tests follow them. Test Guide §3, §4 and §5 make three of these a MUST:

• Naming. Tests are named test_intersect_<mode>_... with no should_ clause, so a failure name states the subject but not the expected outcome.
• Docstrings. All use uppercase GIVEN / WHEN / THEN prose with no leading Test ... summary line, rather than the guide's capitalised-section form.
• AAA comments. None carry # Arrange / # Act / # Assert.

The marker requirement is already satisfied through the module-level pytestmark.

Raised as A8 in the round-1 review of #235 (.sdlc/reviews/issue-#109/review-1.md), where five of five python reviewers flagged it and all five rated it Advisory for the same reason: every test in the module shares the non-conforming form, so converting only the two touched by that PR would leave the file in two styles. The reviewers were unanimous that a split-style file is worse than either style consistently applied.

Scope. Convert the module wholesale in one pass — 13 test functions as of 8b58119. Mechanical, no behaviour change. tests/test_duckdb_iejoin.py is the in-repo reference for the target form.

Worth checking while in there: the review noted that the module's several "identical" interval datasets already disagree on scores, which is the drift that inline-per-test data invites. #235 hoisted one such pair into a partial_overlap_intervals fixture in conftest.py; the remaining inline datasets could follow the same route where two or more tests genuinely share one.