fix: property validation via serde flatten (partial #4)

[vyuan2037]

Partial fix for #4

Changes

• Use #[serde(flatten)] on Declaration to preserve type-specific fields (properties, isAbstract, superType) during JSON deserialization
• Add get_properties() helper on Declaration, remove hardcoded find_properties_for_declaration() workaround
• Validate properties in all concept-like declarations (Concept, Asset, Participant, Transaction, Event)
• Validate scalar bounds: integer range ordering, string length non-negative and minLength ≤ maxLength
• Detect duplicate decorator names on declarations

Flags

• Partial fix for Implement Property Type Dispatch and Validation in [property_type.rs] #4, not closing it yet
• Scalar validator bounds and duplicate decorator checks now implemented
• $class false positive deferred — root cause is a deserialization issue with #[serde(flatten)] and nested $class discriminators, not validation logic

Screenshots or Video

N/A

Related Issues

• Issue Implement Property Type Dispatch and Validation in [property_type.rs] #4

Author Checklist

• Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• Vital features and changes captured in unit and/or integration tests
• Commits messages follow AP format
• Extend the documentation, if necessary
• Merging to main from vyuan2037:fix/property-validation