#2135 bench: warn about expert profile contamination (#2088)

Description

Fixes #2088.

## Summary
Warn when benchmarking could contaminate a learned expert profile.

## Changes
- Detect `expert_profile_save` and `--expert-profile-save` in the selected config.
- Warn even with `--quiet`.
- When using `--url` without `--config`, explain that the attached server's save settings cannot be inspected.
- Document fixed-profile benchmarking and add regression tests.

No server lifecycle changes: the benchmark does not stop servers it did not start.

## Validation
- Ubuntu / Python 3.12: **16/16 regression tests passed**.
- Tested file contents match this PR branch.
- No GPU hardware testing was performed.

GitHub Actions: https://github.com/nekomario28/Strata/actions/runs/38272872006