test: add adversarial guardrail fixtures

[kholdrex]

Summary

Adds a fixture-driven adversarial guardrail corpus for CodeToQuery. The new coverage exercises SQL linter rejection paths and validator allowlist/policy enforcement for common bypass patterns, while hardening validator key access so policy checks apply consistently after schema validation.

Changes

• Adds YAML adversarial fixtures for SQL injection and intent allowlist bypass scenarios.
• Drives new SqlLinter and Validator specs from those fixtures.
• Updates Validator allowlist enforcement to read string- and symbol-keyed hashes consistently.
• Adds regression coverage for default EXISTS filter columns using the validated intent key style.

Test plan

• Static diff check passed.
• YAML fixture files were parsed and category coverage was verified locally.
• Independent GPT and Claude Code reviews completed with no blocking findings.
• Full Ruby/RSpec/RuboCop execution is deferred to GitHub Actions because Ruby and Bundler are not installed on this host.