Escape surrogate values in tracing output

[Himanshuagrawal4]

Summary

• Use
  epr() for hook kwarg values in trace output so surrogate characters are escaped safely.
• Pre-format hook return values before writing tracing output while keeping structural trace labels readable.
• Add regression tests for surrogate values in hook arguments and return values.

Tests

• git diff --check
• PYTHONPATH=src python -m pytest testing\test_tracer.py::test_dictargs_escape_surrogate_values testing\test_pluginmanager.py::test_hook_tracing_escapes_surrogate_values -q (2 passed)
• PYTHONPATH=src python -m pytest testing\test_tracer.py testing\test_pluginmanager.py -q (49 passed)

Fixes #681.