fix: tighten python fuzzing examples

[berisher]

Summary

This PR finishes the Python-side cleanup around the mutator/example bundle now
that the underlying generator reset behavior already landed in the earlier
generator-contracts work.

• stop logging every parser exception from fuzz_pickle_parser()
• fix the shipped example harness so it compiles and shows a safe placeholder shape
• update the PickleMutator docstring and README Atheris snippets to avoid
  long-lived copy-paste patterns and unsandboxed pickle.loads() guidance
• add Python regression tests for reusable mutators, no-logging parser failures,
  and example harness syntax

Remediated Findings

• F-0016
• F-0017
• F-0055
• F-0064
• F-0065
• F-0066

How to validate

uv run ruff check python/pickle_fuzzer/fuzzer.py python/examples/harness.py python/tests/test_fuzzer.py
python3 -m py_compile python/pickle_fuzzer/fuzzer.py python/examples/harness.py python/examples/ci-harness.py python/tests/test_fuzzer.py
uv run pytest python/tests/test_fuzzer.py python/tests/test_generation.py