perf(supervisor-network): avoid reparsing native policy input

[shiju-nv]

Summary

OpenShell currently builds a serde_json::Value, serializes it to JSON text, and asks Regorus to parse that text into its own value representation. Convert the existing serde_json::Value directly into regorus::Value, then pass it to Engine::set_input. This removes the intermediate string and JSON text parse while continuing to return conversion errors before policy evaluation.

Changes

• Rename the constructed native-policy input from input_json to input.
• Convert the existing serde_json::Value directly into regorus::Value without serializing and reparsing JSON text.
• Return conversion errors before policy evaluation instead of allowing Regorus's infallible From<serde_json::Value> conversion to map failures to Undefined.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

[mrunalp]

Do we want to audit and address similar call sites in opa.rs as well?