Handle broken repr in tracing

[ShipItAndPray]

Summary

• guard tracing stringification so hook tracing does not break hook execution when traced objects have a broken __repr__/__str__
• keep the fallback output informative by using the default object repr and the original exception
• add a regression test covering broken repr values in traced hook kwargs

Testing

• uv run --python 3.12 pytest testing/test_pluginmanager.py -k broken_repr -q
• uv run --python 3.12 pytest testing/test_pluginmanager.py -q

Closes #424.