fix(rust): add serde flatten for subclass field preservation

[vyuan2037]

Related to accordproject/concerto-rust#4

Changes

• Add #[serde(flatten)] with catch-all extra field to Rust codegen for base types that have subclasses (Declaration, Property, ConceptDeclaration, etc.)
• When deserializing Concerto JSON AST, parent structs now preserve child-specific fields instead of silently dropping them
• Add serde_json and HashMap imports when base types are present in the generated output

Flags

• Cross-repo change: the generated concerto_metamodel_1_0_0.rs in concerto-rust is produced by this codegen. A companion PR in concerto-rust will use the updated output.
• All 569 existing tests pass, snapshot updated automatically

Screenshots or Video

N/A

Related Issues

• Implement Property Type Dispatch and Validation in [property_type.rs] concerto-rust#4 (property type validation)
• fix: property validation via serde flatten (partial #4) concerto-rust#7 (companion PR -- property validation fix)

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/rust-serde-flatten