Description

This PR fixes a stats contract violation in DEMetropolis that can break population sampling output handling and trigger CI failures like:

• ValueError: setting an array element with a sequence

Root cause:

• DEMetropolis.stats_dtypes_shapes declares scalar stats (tune: bool, scaling: float64),
• but DEMetropolis.astep was emitting:
  • tune as str | None
  • scaling as a NumPy array

This mismatch can propagate into downstream stats packing/trace handling and fail tests in tests/sampling/test_population.py.

Fix

In DEMetropolis.astep stats payload:

• "tune": self.tune is not None (bool)
• "scaling": float(np.mean(self.scaling)) (scalar float)

This restores consistency with stats_dtypes_shapes and aligns behavior with DEMetropolisZ (which already reports scalar scaling).

Fixes #8169

Testing

Targeted population sampler checks:

$env:PYTENSOR_FLAGS='cxx='
.\.venv\Scripts\python.exe -m pytest tests/sampling/test_population.py -k "checks_population_size or demcmc_warning_on_small_populations or nonparallelized_chains_are_random or parallelized_chains_are_random" -q

Observed result relevant to this PR:

• The original ValueError: setting an array element with a sequence is no longer reproduced.
• test_checks_population_size: pass
• test_demcmc_warning_on_small_populations: pass