Fix transition sampler count handling

[FlorianPfaff]

Summary:

• Preserve unary transition samplers for n=1.
• Raise ValueError when n is not 1 and the callback has no sample-count argument.
• Add regression tests for SampleableTransitionModel and DensityTransitionModel.

Tests: Not run locally.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	48.18s
✅ JSON	prettier	7	0	0	0	0.7s
✅ JSON	v8r	7		0	0	4.01s
✅ MARKDOWN	markdownlint	68	0	0	0	1.39s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.75s
✅ PYTHON	black	1036	38	0	0	40.06s
✅ PYTHON	isort	1036	56	0	0	6.93s
✅ REPOSITORY	checkov	yes		no	no	43.46s
✅ REPOSITORY	gitleaks	yes		no	no	8.54s
✅ REPOSITORY	git_diff	yes		no	no	0.1s
✅ REPOSITORY	secretlint	yes		no	no	25.87s
✅ REPOSITORY	syft	yes		no	no	4.39s
✅ REPOSITORY	trivy-sbom	yes		no	no	4.05s
✅ REPOSITORY	trufflehog	yes		no	no	19.85s
✅ YAML	prettier	11	0	0	0	0.58s
✅ YAML	v8r	11		0	0	8.2s
✅ YAML	yamllint	11		0	0	0.35s

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