Preserve dispersion regularization in optimizer fallback

[Intron7]

When dispersion optimization fails, fit_alpha_mle calls grid_fit_alpha without forwarding its regularization settings. The grid therefore enables Cox-Reid correction and disables the dispersion prior regardless of the requested objective. In particular, MAP fallback silently drops its prior.

Forward prior_disp_var, cr_reg, and prior_reg so fallback continues minimizing the configured objective. Default gene-wise fallback behavior and the unsuccessful convergence flag are unchanged.

The regression forces optimizer failure for BFGS and L-BFGS-B across all four Cox-Reid/prior combinations. It compares the result with an independently minimized negative-binomial log-PMF plus the requested penalties. Six cases fail before the fix; all eight pass afterward.

Validation: full upstream suite passes (73 tests); Ruff lint, formatting, and mypy pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.05%. Comparing base (95ace12) to head (485dfae).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #471   +/-   ##
=======================================
  Coverage   86.05%   86.05%           
=======================================
  Files          15       15           
  Lines        1276     1276           
=======================================
  Hits         1098     1098           
  Misses        178      178           

Files with missing lines	Coverage Δ
src/pydeseq2/dispersions.py	96.84% <ø> (ø)

... and 2 files with indirect coverage changes

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