Avoid mutating dicts in jsondumps

[RaghunandanKumar]

Summary

• avoid mutating caller-owned dictionaries when jsondumps(..., exclude_keys=...) is used
• ignore excluded keys that are not present, matching filtering behavior instead of raising KeyError
• add a focused regression test for both cases

Why

jsondumps previously deleted excluded keys directly from dictionary inputs before encoding. That produced surprising side effects for callers that reused the same object after serialization, and it also raised when an excluded key was absent.

Verification

• OPENAI_API_KEY=dummy uv run --python 3.11 pytest evals/data_test.py evals/record_test.py tests/unit/evals/test_metrics.py

[RaghunandanKumar]

Replaced by #1678 with a cleaner branch name.