Preserve Gaussian subclasses in conversion factory

[FlorianPfaff]

Summary

Fix GaussianDistribution.from_distribution(...) so the target-centric convert_distribution(source, TargetSubclass) API actually returns the requested Gaussian subclass.

Bug

The conversion gateway dispatches to TargetSubclass.from_distribution(...). GaussianDistribution.from_distribution was a staticmethod and explicitly constructed GaussianDistribution, so subclasses inheriting that factory silently lost their requested concrete type.

The same problem affected the special GaussianMixture branch because to_gaussian() returns the base Gaussian representation.

For example, converting a valid Gaussian source to a subclass of GaussianDistribution returned a plain GaussianDistribution even though the requested target type was the subclass.

Fix

• convert GaussianDistribution.from_distribution to a classmethod
• construct ordinary conversions through cls(...)
• preserve the existing base-class GaussianMixture.to_gaussian() path exactly
• reconstruct mixture conversions through cls(...) only when a subclass was explicitly requested
• retain the existing check_validity behavior

Regression coverage

Added focused tests covering both factory branches:

1. GaussianDistribution -> GaussianSubclass
2. GaussianMixture -> GaussianSubclass

Both regressions verify the exact requested concrete type and preservation of the resulting mean/covariance.

Scope

• branch is based directly on current main at baac3b1736bb8e81eafc1e3f69e10104636103b9
• 2 commits ahead, 0 behind
• production change is confined to the Gaussian conversion factory
• no overlap with open PR Preserve custom distribution subclasses in conversion factories #5357, which fixes the analogous issue for custom distribution factories

Full repository/backend validation is delegated to GitHub Actions because this execution environment cannot clone/install the repository from GitHub.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Max errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no		no	6.83s
✅ JSON	prettier	7	0	0		0	0.66s
✅ JSON	v8r	7		0		0	5.23s
✅ MARKDOWN	markdownlint	68	0	0		0	2.52s
✅ MARKDOWN	markdown-table-formatter	68	0	0		0	0.62s
✅ PYTHON	black	1989	15	0		0	66.03s
✅ PYTHON	isort	1989	27	0		0	2.94s
✅ REPOSITORY	betterleaks	yes		no		no	1.35s
✅ REPOSITORY	checkov	yes		no		no	35.03s
✅ REPOSITORY	git_diff	yes		no		no	0.14s
✅ REPOSITORY	secretlint	yes		no		no	104.01s
✅ REPOSITORY	syft	yes		no		no	2.65s
✅ REPOSITORY	trivy-sbom	yes		no		no	2.87s
✅ YAML	prettier	11	0	0		0	0.7s
✅ YAML	v8r	11		0		0	17.06s
✅ YAML	yamllint	11		0		0	0.82s

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