capture: make request records privacy-safe by default

[PhilipJohnBasile]

This extracts only the request-capture privacy boundary from #336.

Request captures are still off unless MTPLX_REQUEST_CAPTURE_DIR is set. Once enabled, the default record now replaces prompt and completion token IDs with counts and stable SHA-256 digests. Prompt text, response text, messages, and exception text are also omitted by default, while nested credentials are redacted.

The public completion_token_ids(tokens) helper and its existing one-argument call remain supported. It now returns the completion count and digest by default. Raw completion IDs require their own MTPLX_REQUEST_CAPTURE_INCLUDE_COMPLETION_TOKENS=1 opt-in. That flag is separate from MTPLX_REQUEST_CAPTURE_INCLUDE_PROMPT_TOKENS=1, and both paths enforce independent limits.

Measurement

Command: python scripts/measure_request_capture.py --records 1000

Measured source: fe5b5e59a14e0066f3191951483e3a65b9c55592 on Apple arm64 with Python 3.14.6.

• Capture disabled: 0.658 ms total, 0.658 microseconds per record
• Default privacy capture: 595.733 ms total, 595.733 microseconds per record
• Throughput: 1,678.604 records/s
• Output: 1,000 files, 1,349,890 bytes total, 1,349.89 bytes per record
• Payload per record: 256 prompt tokens, 64 completion tokens, 736 prompt characters, 400 response characters

The measurement verifies all 1,000 records completed, raw prompt and completion IDs were absent, prompt/response/message/secret content was absent, and token counts plus digests were present. Validation runs outside the timed section. The committed receipt is request-capture-1000.json.

Validation

• python -m pytest tests/test_request_capture.py -q: 18 passed
• python -m compileall -q mtplx/request_capture.py scripts/measure_request_capture.py tests/test_request_capture.py: passed
• python -m ruff check mtplx/request_capture.py scripts/measure_request_capture.py tests/test_request_capture.py: passed
• git diff --check origin/main...HEAD: passed
• U+2014 and U+2013 scan across all four changed files: no matches

Exclusions

This PR does not change server call sites, runtime registries, replay systems, dashboards, API routes, workflow files, model behavior, or scheduling. The measurement covers request-capture persistence overhead only. It does not claim inference throughput or model-quality improvement.