test: add regression coverage for public tasks

[timmaundu]

Summary

• Add regression coverage for task cases that should reject cross-account access.
• Add coverage for timeout recovery where only the single expected record should be reported.
• Keep the assertions tied to the public task fixtures so future edits preserve the expected behavior.

Testing

• python -m pytest tests/test_community_identity_tasks.py tests/test_validation.py
• python -m agent_capability_benchmark validate tasks/public