Validate particle resampling sample count

[FlorianPfaff]

Summary

• Validate particle_resampling scenario num_samples as a positive integer before sampling.
• Reject zero, negative, fractional, and boolean sample counts instead of producing empty/NaN estimates or relying on backend-level errors.
• Add regression coverage for invalid num_samples TOML values.

Bug

Before this change, num_samples = 0 reached sampled_mean = ... / float(num_samples), yielding a degenerate non-finite scenario estimate instead of a clear configuration error. Fractional values were also silently truncated via int(...).

Testing

• Added focused regression coverage in tests/test_scenario_registry_and_reproducibility.py.
• Not run locally; this environment cannot clone GitHub or install the repository dependencies, so CI should run the tests on the PR.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
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✅ JSON	prettier	7	0	0	0	0.84s
✅ JSON	v8r	7		0	0	4.86s
✅ MARKDOWN	markdownlint	68	0	0	0	1.51s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.59s
✅ PYTHON	black	890	10	0	0	49.32s
✅ PYTHON	isort	890	11	0	0	2.96s
✅ REPOSITORY	checkov	yes		no	no	43.35s
✅ REPOSITORY	gitleaks	yes		no	no	10.01s
✅ REPOSITORY	git_diff	yes		no	no	0.13s
✅ REPOSITORY	secretlint	yes		no	no	26.92s
✅ REPOSITORY	syft	yes		no	no	4.02s
✅ REPOSITORY	trivy-sbom	yes		no	no	7.15s
✅ REPOSITORY	trufflehog	yes		no	no	21.88s
✅ YAML	prettier	11	0	0	0	0.69s
✅ YAML	v8r	11		0	0	9.81s
✅ YAML	yamllint	11		0	0	0.8s

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