Fix strict nested matrix inner-validation shard checks

[FlorianPfaff]

Summary

• validate inner-validation sidecars against outer_test_participant instead of test_participant
• keep outer, selected, and prediction shard coverage checks on test_participant
• add a regression test for multi-participant shards whose inner-validation rows use a different inner validation participant

Context

PR #155 added the sharded nested stimulus matrix workflow. Later strict aggregation hardening checks that every file in a multi-participant shard has rows for every outer participant encoded in the shard filename. Real inner-validation rows encode the outer fold in outer_test_participant; test_participant is the inner validation fold. The strict checker was reading test_participant for every sidecar, which could reject valid shard artifacts.

Validation

• Inspected current main via the GitHub connector.
• Could not run repository tests locally because this execution environment cannot resolve github.com for a clone/install.
• Added focused unittest coverage for the real inner-validation row shape; CI should run the focused nested-matrix tests and full static checks.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	3.81s
✅ MARKDOWN	markdownlint	13	0	0	0	0.89s
✅ PYTHON	ruff	128	0	0	0	0.28s
✅ REPOSITORY	git_diff	yes		no	no	0.02s
✅ YAML	prettier	6	0	0	0	0.49s
✅ YAML	v8r	6		0	0	3.49s
✅ YAML	yamllint	6		0	0	0.46s

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