Use stable logsumexp in grid plots

[fujikix1102]

Summary

• replace direct log(exp(like).sum()) calculations with SciPy's stable logsumexp
• cover both 1D and 2D grid marginalization paths
• add tests for extreme log-likelihoods and empty groups

Validation

• pytest -q cosmosis/test/test_grid_plots.py --disable-warnings --maxfail=1 (2 passed)
• Python compilation and git diff --check passed

[fujikix1102]

The Python 3.9 macOS/Ubuntu jobs terminate after the existing MultiNest/PolyChord/L-BFGS-B sampler output with exit code 1; the available excerpt contains no grid-plots traceback or failing test name. All 3.10+ jobs pass. Could a maintainer please rerun the two 3.9 jobs or expose the final pytest traceback? The dedicated grid-plot tests pass locally.

[fujikix1102]

CI run 35924752621 identified the failure: the new empty-group regression test called logsumexp on an empty array, raising ValueError in Python 3.9. Commit c10ef96 now treats an empty marginalization group as mathematical log-sum-exp = -inf. The two grid plot tests pass locally; please rerun CI.