fix: synthetic PositionsLH under skip_checks + test_mode

[Jammy2211]

Summary

Workspace integration scripts using a chained .positions extraction (e.g. scripts/group/features/pixelization/slam.py:735-737) were failing under the smoke profile (PYAUTO_TEST_MODE=2 PYAUTO_SKIP_CHECKS=1) with AttributeError: 'NoneType' object has no attribute 'positions'. The cause was Result.positions_likelihood_from short-circuiting to None when skip_checks() is true, instead of falling through to the existing test-mode synthetic-positions safeguard.

This change makes the function return a usable PositionsLH (built from the same [(1.0, 0.0), (-1.0, 0.0)] synthetic fallback already used elsewhere in the function) when both skip_checks() and is_test_mode() are active. Production semantics are preserved: with skip_checks alone (no test mode) the function still returns None.

API Changes

autolens.analysis.result.Result.positions_likelihood_from no longer returns None under the combination PYAUTO_SKIP_CHECKS=1 + PYAUTO_TEST_MODE>0 — it now returns a synthetic PositionsLH with positions=[(1.0, 0.0), (-1.0, 0.0)] and threshold=minimum_threshold or 0.5. Behaviour is unchanged when skip_checks alone is active (still returns None) and when neither flag is active (full multiple-image-position computation as before).

See full details below.

Test Plan

• python -m pytest test_autolens/analysis/test_result.py -k positions_likelihood_from -x passes (5 tests, including 2 new)
• autolens_workspace/scripts/group/features/pixelization/slam.py runs end-to-end under the smoke profile

Full API Changes (for automation & release notes)

Changed Behaviour

• Result.positions_likelihood_from — under PYAUTO_SKIP_CHECKS=1 + PYAUTO_TEST_MODE>0, returns a synthetic PositionsLH instead of None. Threshold defaults to minimum_threshold or 0.5 when unset. Plane redshift is forwarded.

Added (tests)

• test__positions_likelihood_from__skip_checks_returns_none_outside_test_mode — confirms the production-path None return is unchanged.
• test__positions_likelihood_from__skip_checks_returns_synthetic_in_test_mode — confirms the new synthetic return under skip_checks + test_mode.

Migration

• No migration needed. Workspace scripts that chain .positions on the return value (e.g. result.positions_likelihood_from(...).positions) now work end-to-end under the smoke profile without per-script guards.

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