Reject non-table scenario sections

[FlorianPfaff]

Bug

A syntactically valid TOML file can define scenario as a scalar or array instead of a table, for example:

scenario = "linear_gaussian"

The scenario dispatcher and both direct scenario runners immediately called .get(...) on that value. This leaked an internal AttributeError instead of rejecting the malformed configuration through the public validation path.

Fix

• add one shared _scenario_section() validator;
• reject non-table scenario values with ValueError("scenario must be a TOML table");
• reuse the validated table for type and name lookup in both built-in runners;
• preserve the existing behavior for absent, empty, and unsupported scenario types.

Regression coverage

The new test exercises all three public entry points against string-, integer-, and array-valued scenario entries (nine cases total).

Validation

• updated module and regression test pass python -m py_compile;
• focused standalone reproduction passed for all nine cases;
• branch is 2 commits ahead of main and 0 behind;
• diff is limited to src/pyrecest/scenarios.py and tests/test_scenario_section_validation.py.

Repository CI should run the full backend and test matrices on this pull request.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	23.61s
✅ JSON	prettier	7	0	0	0	1.05s
✅ JSON	v8r	7		0	0	4.71s
✅ MARKDOWN	markdownlint	68	0	0	0	1.76s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.9s
✅ PYTHON	black	1703	257	0	0	98.29s
✅ PYTHON	isort	1703	482	0	0	2.77s
✅ REPOSITORY	betterleaks	yes		no	no	1.92s
✅ REPOSITORY	checkov	yes		no	no	57.06s
✅ REPOSITORY	gitleaks	yes		no	no	16.23s
✅ REPOSITORY	git_diff	yes		no	no	0.32s
✅ REPOSITORY	secretlint	yes		no	no	77.52s
✅ REPOSITORY	syft	yes		no	no	5.2s
✅ REPOSITORY	trivy-sbom	yes		no	no	6.23s
✅ REPOSITORY	trufflehog	yes		no	no	33.04s
✅ YAML	prettier	11	0	0	0	0.67s
✅ YAML	v8r	11		0	0	11.62s
✅ YAML	yamllint	11		0	0	0.49s

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