Fix CODEOWNERS empty path parsing

[cursor]

Fixes CODEOWNERS schema updates when path mapping collapses a CODEOWNERS path to an empty ownership matcher such as codeowners: #team.

Summary

• Preserve mapped CODEOWNERS root paths as / so conversion never emits codeowners: with an empty matcher.
• Skip legacy/invalid empty codeowners: matchers before invoking the strict grammar parser.
• Preserve ownerless CODEOWNERS rules with non-empty paths as exclusions, including mapped-root exclusions like codeowners:/.
• Add regression coverage for conversion, schema creation, and the update_code_owners_schema task path.

Test Plan

• python3 - <<'PY' ... PY isolated smoke test passed: converted '/apps @octocat\n' to 'codeowners:/ octocat@sentry.io\n', converted the ownerless exclusion '/apps\n' to 'codeowners:/\n', skipped codeowners: #discover, and verified codeowners:/apps/github still parses as a valid ownerless exclusion.
• python3 -m compileall -q src/sentry/issues/ownership/grammar.py tests/sentry/issues/ownership/test_grammar.py tests/sentry/tasks/test_code_owners.py
• git diff --check
• Attempted .venv/bin/pytest -n3 -svv --reuse-db ... but .venv/bin/pytest is unavailable in this cloud workspace.
• Attempted .venv/bin/prek run -q --files ... but .venv/bin/prek is unavailable in this cloud workspace.
• Attempted SENTRY_DEVENV_FRONTEND_ONLY=1 devenv sync but devenv is unavailable in this cloud workspace.
• Attempted make test-selective but it fails before running tests because gcloud is not installed.

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