Skip cross-repo integration test when sibling repos absent (ar-r82f.6)

[atc964]

Summary

tests/integration/test_path_a_audience_e2e.py::TestCrossRepoAudiencePlanJSONRoundTrip::test_cross_repo_audience_plan_json_round_trip was the one remaining failure in CI. It exercises an AudiencePlan JSON round-trip through the seller repo's AudienceRef model and locates the seller src/ either via AD_SELLER_SRC_PATH or by walking up the path ancestry looking for an ad_buyer_system directory with a sibling ad_seller_system.

Standalone clones of IABTechLab/buyer-agent (including the GitHub Actions runner) check out only this repo, in a directory named buyer-agent rather than ad_buyer_system, so the ancestry walk returned None and the test raised RuntimeError: Could not locate ad_buyer_system in path ancestry ... ; set AD_SELLER_SRC_PATH to override. This was pre-existing, but only became visible after the collection error fixed in ar-r82f.5.

This PR keeps the test logic and assertions unchanged. It just gates the cross-repo half on environment availability:

• If AD_SELLER_SRC_PATH is unset and no ad_buyer_system ancestor is found, pytest.skip cleanly with a message naming the env override.
• If an ad_buyer_system ancestor is found but the resolved seller src/ directory does not exist on disk (partial checkout), pytest.skip instead of failing on the import.
• All other code paths (env var set; sibling repos / worktrees present locally) continue to run the test exactly as before.

How to run locally

The test still runs whenever the cross-repo layout or env override is present:

• Check out the repo into a directory named ad_buyer_system/ alongside a sibling ad_seller_system/ containing src/ad_seller/..., or
• Set AD_SELLER_SRC_PATH=/path/to/ad_seller_system/src before invoking pytest.

Then: uv run pytest tests/integration/test_path_a_audience_e2e.py -v

Expected CI impact

This should bring the CI Test job to GREEN (0 failures) for the first time in a long while: the cross-repo round-trip moves from FAILED to SKIPPED, every other integration test in the file continues to run, and the skip count goes up by one.

References

• Bead: ar-r82f.6
• Parent epic: ar-r82f
• Sister test: tests/integration/test_path_b_audience_e2e.py (no cross-repo dependency, already passing)

Test plan

• CI passes (target: 3196 passed, 0 failed, ~60 skipped)
• uv run pytest tests/integration/test_path_a_audience_e2e.py -v locally shows the cross-repo test as SKIPPED on a standalone clone