BST-18566 Add env field to test definition schema

[fproulx-boostsecurity]

Summary

• Add env field to Test model for specifying environment variables in tests.yaml
• Add env field to MatrixEntry model to pass environment variables through matrix expansion
• Update to_matrix_entries() to propagate env vars from Test to MatrixEntry
• Add unit tests for env field parsing and serialization

Context

This is Phase 1 of the E2E test environment variables implementation plan documented in docs/PLAN-e2e-test-env-vars.md. This enables specifying environment variables like BOOST_TF_TAGS_POLICY in tests.yaml files:

tests:
  - name: "aws-with-tag-policy"
    type: "source-code"
    source:
      url: "https://github.com/boost-sandbox/checkov-tf-plan-testing.git"
      ref: "main"
    scan_paths:
      - "aws-security-violations"
    env:
      BOOST_TF_TAGS_POLICY: '{"required_tags":["Environment"]}'

Test plan

• Unit tests for env field passthrough
• Unit tests for env field serialization
• Integration tests updated for new env field in matrix output
• All 161 tests pass with 100% coverage
• Linting passes (mypy + ruff)

🤖 Generated with Claude Code

[lindycoder]

With @Dyc0de we considered having an allow list of prefix to avoid screwing up stuff in ways we can't think of, what do you think?