Harden dataset participant ID parsing

[FlorianPfaff]

Summary

• Reject boolean participants.ids inputs before Python treats them as integers.
• Reject mapping-valued participant specs before they are silently iterated as keys.
• Add regression coverage while preserving compact range/list participant parsing.

Validation

• Branch diff against main: 3 commits ahead, 0 behind.
• Focused tests added in tests/test_dataset_config.py; PR CI should run the suite.

Risk and compatibility

• Valid integer, string, range, and list participant specs are unchanged.
• Ambiguous YAML/JSON booleans and mappings now fail fast with actionable ValueErrors.

[github-actions]

Test Results

    4 files      4 suites   1m 43s ⏱️
  660 tests   651 ✅  9 💤 0 ❌
2 640 runs  2 604 ✅ 36 💤 0 ❌

Results for commit 8479b85.

[github-actions]

✅⚠️MegaLinter analysis: Success with warnings

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	14		0	0	0.41s
✅ COPYPASTE	jscpd	yes		no	no	8.66s
⚠️ MARKDOWN	markdownlint	55	0	1	0	1.7s
✅ PYTHON	ruff	247	0	0	0	0.33s
✅ REPOSITORY	git_diff	yes		no	no	0.04s
✅ YAML	prettier	48	0	0	0	1.67s
✅ YAML	v8r	48		0	0	8.55s
✅ YAML	yamllint	48		0	0	1.04s

Detailed Issues

⚠️ MARKDOWN / markdownlint - 1 error

.github/PULL_REQUEST_TEMPLATE.md:1 error MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "## Summary"]

Notices

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