fix: Aggregator.from_directory test-mode-aware (interpolate tutorial IndexError)

[Jammy2211]

Summary

autofit_workspace's features/interpolate.py crashed with IndexError: Cannot interpolate: no instances have been added to the interpolator under test mode (and full-script sweeps). Root cause: under PYAUTO_TEST_MODE searches write results beneath an inserted test_mode segment (output/test_mode/<prefix>, via _test_mode_segment() in non_linear/paths/abstract.py), but Aggregator.from_directory walked only the real-run path the caller points at (output/<prefix>), found nothing, and returned an empty aggregator — the tutorial's database section then interpolated over zero instances.

The in-memory interpolation section already worked; only the aggregator/database section failed, and only under test mode (real user runs are unaffected). This restores the missing symmetry: the aggregator now resolves the test_mode sibling the way searches namespace their output.

Fixes #1401.

API Changes

None affecting real runs — internal test-mode recovery only. The from_directory signature is unchanged and non-test-mode behaviour is byte-for-byte identical. In test mode only, an empty first scan now retries once against the test_mode sibling directory.
See full details below.

Test Plan

• pytest test_autofit/aggregator/ — 58 pass (incl. 2 new from_directory test-mode tests)
• pytest test_autofit/aggregator/ test_autofit/interpolator/ — 93 pass, no regressions
• autofit_workspace/scripts/features/interpolate.py under PYAUTO_TEST_MODE=1 with this branch — exit 0, aggregator loads 3 outputs, interpolation succeeds

Full API Changes (for automation & release notes)

Changed Behaviour

• autofit.aggregator.Aggregator.from_directory(directory, ...) — when is_test_mode() is active and the first scan of directory yields zero search-outputs, it now retries once against Path(directory).parent / "test_mode" / Path(directory).name if that path exists. Outside test mode, and when the first scan is non-empty, behaviour is unchanged. No signature change; no migration required.

Validation checklist (--auto run — plan was human-approved)

• Effective level: supervised; the two-level plan (incl. Option A) was human-approved before implementation, not merely written to the issue.
• Plan: on issue fix: Aggregator.from_directory is not test-mode-aware (interpolate tutorial IndexError) #1401, unmodified since.
• Gate: tests 58/58 aggregator (93 with interpolator) · smoke n/a (interpolate.py not in the smoke set; validated directly under PYAUTO_TEST_MODE=1) · review CLEAN · Heart RED human-waived — sole RED reason was an unrelated dirty file (PyAutoLens/paper_jax/paper.md WIP), not this change; all YELLOW reasons pre-existing/unrelated.
• Human: plan sound in hindsight?
• Human: diff matches plan (no scope creep)?
• Human: merge, amend, or reject — then log the outcome

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