fix: make the PointSolver SMALL_DATASETS short-circuit announce itself

[Jammy2211]

Closes #710 (library leg). The workspace leg is PyAutoLabs/autolens_workspace_test#265 and must merge after this one.

What this fixes

Under PYAUTO_SMALL_DATASETS=1, PointSolver.solve skips the triangle-tiling solve and returns the fixed pair [(1.0, 0.0), (0.0, 1.0)]. That is intentional — the solve dominates smoke-run cost and is meaningless on downsized grids — but it was completely silent, and the pair is identical for every lens model.

Anything derived from those positions is therefore model-independent, so a parity script comparing such a value against a pinned literal is measuring nothing. That is exactly how it surfaced: autolens_workspace_test's point_source/jax_likelihood/point.py produced a different wrong value on every run — once the -1e99 sentinel, once a finite-but-wrong 16.131221 — and cost a full investigation before the cause was found.

The change

The short-circuit now emits one logger.warning per process, latched at module level rather than per instance (a vmap batch calls solve once per sampled parameter set, and the condition is about the process environment, not any one solver).

Warn rather than raise on purpose: every smoke script that legitimately relies on the speedup keeps working, and only the silence goes away.

Evidence

Measured in-session against libraries 2026.8.23.1 + this branch:

Check	Result
solve() at einstein_radius 1.0 / 1.6 / 2.5 under the cap	identical [(1.0, 0.0), (0.0, 1.0)]
log output at DEBUG, warnings forced to always, before this change	nothing at all

API Changes

None. No signature, return type or default changes — the short-circuit returns exactly what it returned before. The only new behaviour is one WARNING log record per process on the autolens.point.solver.point_solver logger.

Tests

Three added to test_autolens/weak/test_simulator_small_datasets.py, covering each property independently:

• the returned pair is model-independent across three Einstein radii
• the warning fires exactly once across repeated solve calls
• nothing is emitted when the flag is unset

test_autolens/weak/test_simulator_small_datasets.py  6 passed
test_autolens/                                     541 passed

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