docs(search): state the opt-in rule for _test_mode_samples_info

[Jammy2211]

Summary

NonLinearSearch._test_mode_samples_info() told subclasses to override it "so
that tutorial scripts and downstream code can access those keys without
KeyError" — which reads as a per-sampler obligation. Nine search modules write
samples_info in their real path (nautilus, dynesty, emcee, zeus, bfgs, drawer,
blackjax nuts, multi_start_gradient, abstract) and exactly one overrides the
hook, so the other eight look like they are missing one.

They aren't. This PR documents the rule the existing decisions actually follow.

Reachability. No library path reads these diagnostic keys under bypass. The
properties that read them — SamplesMCMC.total_steps (samples/mcmc.py:200),
SamplesNest.number_live_points / total_samples / log_evidence
(samples/nest.py:77-92) — live on Samples subclasses the bypass never
constructs; _fit_bypass_test_mode always builds a SamplesPDF. The only
consumers are workspace scripts reading samples_info[...] directly.

Consumer sweep across all eleven workspace/tutorial repos finds exactly two:

Site	Keys	Runs bypassed?
autofit_workspace/scripts/searches/mcmc.py:335	ess_min, num_samples, mean_acceptance, n_divergent, n_logl_evals	yes — no __Env__, and it is line 2 of that workspace's smoke_tests.txt, so it runs bypassed on every PR
autofit_workspace_test/.../multi_start_gradient_auto_convergence.py:130	total_steps	no — declares ENV: real_search jax

The first is why the NUTS override exists (#1260). The second was fixed on the
workspace side (autofit_workspace_test#83 / PR#84, merged f4c45c1). Nautilus,
Dynesty, Emcee and Zeus have no bypassed consumer either — hence no override.
autofit_workspace/scripts/searches/mle.py uses MultiStartAdam but never
touches samples_info, so MultiStartGradient has no bypassed consumer at all.

The rule, now in the docstring: prints → placeholders; asserts → real search.
A tutorial that only displays diagnostics may legitimately run bypassed, so its
search needs the override with honest empties. A script that asserts on them
must not run bypassed at all — it declares ENV: real_search. Adding
placeholders for an asserting reader is worse than the KeyError it replaces,
because the assert then silently passes on a stub value.

API Changes

None. Docstring-only — no signature, behaviour or output change.

Rejected alternatives

• Add an AbstractMultiStartGradient override for surface consistency with
  NUTS: it serves no existing consumer, and a placeholder total_steps would let
  a future assert total_steps < n_steps silently pass on a stub 0 — the exact
  failure mode avoided in autofit_workspace_test#83.
• A unit test pinning the bypass keys: a test can pin BlackJAXNUTS's keys
  but cannot express "and the others deliberately have none" without freezing the
  sampler roster.

Test Plan

• pytest test_autofit/non_linear/search — 172 passed, 1 skipped
• Docstring renders and is reachable at runtime
  (NonLinearSearch._test_mode_samples_info.__doc__)
• Diff is one file, docstring only

Closes #1448

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