#2605 bench: a bench-started server also drops the config's "expert_profile_save" key, not only the argument (#2088)

Descripción

On #2088; the gap docs PR #2601 describes.

**What it changes.** When the bench starts the server itself, `without_profile_save()` in `tools/strata_bench.py`
gives it a copy of the config without `--expert-profile-save`, so the bench's short prompts do not overwrite the
user's profile (017a672a). But `serve/server.py` `learned_profile_args()` adds that same argument back from a
config's `"expert_profile_save"` key (#477, opt-in): a bench-started server from such a config still saved its
profile at QUIT and every 10 minutes, and started from the learned profile instead of the config's own
`--expert-profile`, against the pinned-profile rule in docs/COMMUNITY_BENCHMARKS.md. The copy now also leaves out
`"expert_profile_save"` and `"expert_profile_save_every"`, and the stderr note names the key.

**What it leaves alone.** The user's config file (only the temporary copy beside the bench log changes). A config
with neither the argument nor the key: the same object, started from the user's own file as before. `--url` runs
(the bench does not start that server). The server's handling of the key.

**How it was checked.** `tools/test_strata_bench.py`
`ReportTests.test_a_bench_server_never_saves_over_the_users_profile` now also covers a config with the key: the copy
has no key, `learned_profile_args()` on the copy adds no `--expert-profile-save` (on the original it does), and
`start_server` writes the copy without the key. Fails on `main`; 17 tests OK. Python 3.14, Ubuntu (Linux 7.0); no GPU.

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