fix(policy): name the field in policy type errors

[ericcurtin]

Summary

Policy type errors now name the bad field, like unknown-field errors do.

Related Issue

Closes #4168

Changes

• Decode with serde_path_to_error (already in the lockfile) and prefix the error with the field path.
• Share the path length cap with unknown-field errors.

Testing

• Checks appropriate to the affected code and behavior pass
• Unit tests added/updated (if applicable)
• E2E tests added/updated (if applicable)

cargo fmt, cargo clippy -D warnings and cargo test for openshell-policy-schema and openshell-policy. Prover tests not run (no local Z3).

Checklist

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

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[ericcurtin]

@mrunalp @johntmyers PTAL when you get a chance, and /ok to test f61db311e if it looks good. Thank you!