util for enforcing env vars are set

[willccbb]

Description

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

---

Note

Introduces a central vf.ensure_keys() and MissingKeyError for validating required environment variables and integrates them across several environments.

• Adds verifiers.utils.config_utils with ensure_keys and MissingKeyError; re-exports in verifiers/__init__.py and documents in docs/reference.md
• Enforces required API keys in continuation_quality, toxicity_explanation, mcp_search_env, and wiki_search via vf.ensure_keys() and switches to strict os.environ[...] access
• Updates environment docs: adds "Required API Keys" guidance in docs/environments.md and environments/AGENTS.md; expands experimental env details; updates READMEs (wiki_search, self_reward) and minor tweaks in AGENTS.md

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