fix(ipc): drop stale ring on failed re-registration

[WilliamK112]

Summary

• remove the stale connector ring entry before closing the old ring during re-registration
• leave the connector without a registered ring if opening the replacement shared-memory segment fails, instead of retaining a closed ring in _ring_registry
• add a regression test for the failed-reopen path that does not depend on an IPC socket fixture

Fixes #209.

Validation

Rebased onto current NVIDIA/xr-ai@37822349729f9c67f492f9aac29b7eeb803d1dd9 on 2026-08-09. The only conflict was the upstream SDK rename from xr_ai_agent to xr_ai_hub; the refreshed test uses the current package.

• focused failed-reopen regression: 1 passed
• complete tests/test_participant_events.py: 5 passed
  • run with a short TMPDIR on macOS to stay below the Unix-domain socket path limit
• Ruff 0.15.16 on both changed Python files: passed
• repository SPDX checker on both changed files: passed
• DCO sign-off present
• git diff --check upstream/main...HEAD: passed

[WilliamK112]

Refreshed this PR onto current main (37822349) at head e6f8d5f4. The only conflict was the upstream SDK rename, so the regression test now imports ConnectorRegistration from xr_ai_hub. Current validation: all 5 participant-event tests pass with a short macOS TMPDIR; focused regression, Ruff 0.15.16, SPDX, DCO, and diff checks are clean.

[WilliamK112]

Refreshed again onto current main (6de6ca73) after XR-Media-Hub moved from server-runtime/ to services/. Git followed the rename cleanly; the PR remains one signed commit and the diff is still limited to the registry pop plus its regression test. GitHub now reports behind_by: 0, ahead_by: 1, and mergeable. Validation on head ed61f3f2: all 5 test_participant_events.py tests pass, Ruff 0.15.16 passes, SPDX and DCO checks pass, and git diff --check is clean.

[WilliamK112]

Refreshed onto current main (b641bf5) at signed head 13cb7135. The six upstream commits replayed cleanly, including the concurrent xr-ai-hub-client changes; the PR diff remains limited to the stale-ring pop and its regression test. Validation: all 5 test_participant_events.py tests pass, Ruff passes, both changed files pass the SPDX checker, the commit retains its DCO sign-off, and git diff --check is clean. GitHub reports the PR mergeable.