fix: support recursive coverage and split postprocessing in pytest_main

[xangcastle]

Two related coverage failures in pytest.py.tmpl: first, when multiple pytest entry points run in the same process (e.g. ROS launch tests), a coverage.Coverage instance is already active when the template is imported a second time.

Creating a fresh one on top silently drops accumulated data, so we now check Coverage.current() and reuse the existing instance instead of starting a new one. Second, when --experimental_split_coverage_postprocessing is enabled Bazel sets COVERAGE_DIR and runs a separate postprocessor action that expects to find the raw coverage data at $COVERAGE_DIR/python_coverage.dat.

Previously the output was always written to COVERAGE_OUTPUT_FILE which the postprocessor never reads, so coverage was silently discarded, the fix prefers COVERAGE_DIR when it is set.

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Changes are visible to end-users: no

Test plan

• New test cases added

[aspect-workflows]

✨ Aspect Workflows Tasks

📅 Thu Jun 11 12:41:04 UTC 2026

❌ 3 failed tasks

• ❌ test (test-e2e-bazel-9) · ⏱ 33m 4s · 🐙 GitHub Actions · ☑️ Check
  💬 failed in test · Bazel test failed (1 test failed)
• ❌ test (test-root-bazel-8) · ⏱ 35m 18s · 🐙 GitHub Actions · ☑️ Check
  💬 failed in test · Bazel test failed (1 test failed)
• ❌ test (test-root-bazel-9) · ⏱ 37m 32s · 🐙 GitHub Actions · ☑️ Check
  💬 failed in test · Bazel test failed (1 test failed)

✅ 3 successful tasks

• ✅ buildifier · ⏱ 30.7s · 🐙 GitHub Actions · ☑️ Check
  💬 Format complete (clean)
• ✅ test (test-e2e-bazel-8) · ⏱ 33m 27s · 🐙 GitHub Actions · ☑️ Check
  💬 Bazel test complete (61/61 passed)
• ✅ test (test-examples-uv_pip_compile-bazel-8) · ⏱ 27.1s · 🐙 GitHub Actions · ☑️ Check
  💬 Bazel test complete (1/1 passed · 1 cached)

🔁 Reproduce

❌ test (test-e2e-bazel-9)

aspect test -- //cases/uv-no-sdist-754:test

# without Aspect CLI
bazel test -- //cases/uv-no-sdist-754:test

❌ test (test-root-bazel-8 · test-root-bazel-9)

aspect test -- //examples/pytest:coverage_dir_test

# without Aspect CLI
bazel test -- //examples/pytest:coverage_dir_test

_{Install aspect: docs.aspect.build/cli/install}

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