test(redteam): cover RedTeamCase metadata sync and _build_system_prompt assembly

[zeroshotmind]

Description

Two small pieces of experimental/redteam logic had no direct unit tests — only incidental exercise as fixture setup inside other strategy test files, which never asserted their actual behavior:

• RedTeamCase._sync_metadata_from_config (case.py): the model_validator that merges AttackGoal fields into metadata. Untested branches: metadata=None gets fully populated from attack_goal; caller-provided keys are preserved (setdefault semantics — config never overwrites an existing key); partial metadata is backfilled for missing keys only; and an explicit empty dict (falsy but not None) still takes the "existing metadata" branch rather than the "populate" branch.
• _build_system_prompt (strategies/_common.py): the helper that assembles the six shared prompt blocks (ROLE_BLOCK, PROFILE_BLOCK, playbook, RULES_BLOCK, EXIT_BLOCK, FORMAT_BLOCK) used by every prompt-based strategy. Untested: that all blocks are present, in the correct order (block order is load-bearing for the {actor_profile}/{max_turns} placeholders filled in later via .format()), joined with a blank line, and that those placeholders survive assembly unconsumed.

This is a test-only PR — no production code changes. Adds tests/strands_evals/experimental/redteam/test_case.py and tests/strands_evals/experimental/redteam/test_common.py.

Related Issues

None — found via a review of test coverage gaps in experimental/redteam/, not filed as an issue first since it's test-only and additive.

Documentation PR

N/A

Type of Change

Tests only

Testing

• Ran python -m pytest tests/strands_evals/experimental/redteam/ — 326 passed (previously 319; +7 new)
• ruff format --check and ruff check clean on the new files
• mypy clean on the new files

Checklist

• I have read the CONTRIBUTING document
• I have reviewed and understand every line of code in this PR, including any generated by AI tools, and I can explain why it works
• My change is focused and reasonably small; I have split unrelated work into separate PRs
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

[pgrayy]

For consistency, I would recommend placing this under the directory redteam/strategies/. This also helps directly map the test module to its src module.