Summary

• Plugin manager: Added _prevalidate_entrypoints() which enforces a single entry point per plugin, records failures as FailedPluginInfo, and blocks broken entry points from being registered in pluggy
• API: GET /api/v1/plugins now returns failed_plugins alongside plugins so the frontend can surface errors
• Frontend: PluginsTab shows a warning banner and per-plugin error details when plugins fail to load
• Registry: Wraps plugin pipeline loading in try/except so built-in pipelines survive plugin failures
• Tests: 8 new tests covering error handling, blocking, failure recording, and pipeline registration resilience

Test plan

• Run uv run pytest tests/test_plugin_manager.py — all 8 new tests pass
• Install a plugin with a broken entry point (e.g. bad import) and confirm it appears in the failed plugins UI
• Confirm healthy plugins still load and built-in pipelines are unaffected

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