Fix surrogate values in hook tracing

[almondsun]

Summary

• render traced hook argument values with repr while preserving structural labels
• pre-format traced hook results with repr at the PluginManager boundary
• cover surrogate-containing kwargs and first-result hook returns
• add a bug-fix changelog fragment

Closes #681.

Root cause and impact

Tracing formatted arbitrary Python values with str, which could leave literal
surrogate characters in the resulting message. Writers using ordinary UTF-8
encoding then raised UnicodeEncodeError. Applying repr only at the two value
positions safely escapes surrogates without quoting trace labels or double-
representing unrelated structural content.

Validation

• uv run pytest testing/test_tracer.py testing/test_pluginmanager.py::test_hook_tracing testing/test_pluginmanager.py::test_hook_tracing_safely_formats_surrogate_result — 8 passed
• uv run pytest — 141 passed
• uv run pre-commit run -a — all hooks passed, including Ruff, Flake8, MyPy, formatting, and ReST checks
• git diff --check

I used Codex as an implementation assistant, reviewed and validated the complete
change, and take responsibility for the contribution and follow-up.

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[yangfan-yf-yf]

Reviewed b7179b9 independently.

The change keeps structural trace labels on str() while applying repr() only to arbitrary traced values, so surrogate characters are escaped without changing the label format. The two regression tests cover both kwargs and hook-result paths.

Validation on Windows with Python 3.12:

• python -m pytest ? 141 passed
• git diff --check ? clean

The GitHub test matrix and pre-commit checks are also passing.