Implement Display for nested shapes referenced in error messages

[drganjoo]

This PR addresses the issue where shapes annotated with @error can reference other complex shapes in their message fields, which previously lacked Display implementations.

Changes

• Added a model transformation (AddSyntheticTraitForImplDisplay) to identify all shapes reachable from error shapes
• Added a synthetic trait to mark shapes that need Display implementations
• Modified code generation to implement Display for all shapes reachable from error shapes
• Ensured consistent formatting for both optional and non-optional fields
• Added tests to verify correct Display implementation for nested error structures

Implementation Details

• The transformation uses a directed graph walker to find all shapes reachable from error shapes
• Only adds implementations to shapes that don't already have the @error trait
• Formats all fields consistently, including handling of Option<T> values
• Maintains the standard "StructName { field=value }" format across all generated Display implementations

Testing

• Added test cases for various error structures with different levels of nesting
• Verified that both simple and complex error shapes correctly implement Display
• Confirmed proper handling of optional fields

Fixes #4080

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