Fuzz test for policy DSL parser

[cobusgreyling]

Summary

Add fuzz/property tests for the backpressure policy YAML/DSL parser to catch parse regressions.

Acceptance criteria

• Fuzz or property tests for invalid/malformed policy inputs
• Valid policy fixtures still parse correctly
• Integrated into pnpm test for packages/policy
• No new runtime dependencies unless justified

References

• packages/policy/src/