feat: add new errors and checks to contract to validate logic

[JoshdfG]

Summary

Describe the change, the motivation behind it, and any important implementation details.

Closes #1662
Closes #1663
Closes #1664
Closes #1665

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Maintenance or refactor

Testing

List the commands you ran and the scope of validation.

cargo fmt --all --check
cargo clippy --workspace --all-targets --all-features -- -D warnings
cargo test -p sanctifier-core --all-features
cargo test -p sanctifier-cli
cd frontend && npm test

Checklist

• I ran the relevant tests locally, or explained why they were not needed.
• I updated documentation for any user-facing behavior changes.
• I added or updated tests for the change when appropriate.
• I added a changelog or release-notes entry when needed, or confirmed none is required.
• I verified this branch is up to date with main and merge conflicts are resolved.

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