fix(source-generators): improve nested property fallback handling for CS8601 warnings

[ncipollina]

🚀 Pull Request

📋 Summary

This PR fixes CS8601 "Possible null reference assignment" warnings by implementing smart fallback logic for nested properties based on their characteristics:

Property Type	Fallback
required + non-nullable	throw new System.InvalidOperationException(...)
Nullable (?)	null
Non-nullable, non-required collection	[] (empty collection)
Non-nullable, non-required object	null

Key Changes

• PropertyMappingCodeRenderer.cs: Added GetNestedObjectFallback() and GetNestedCollectionFallback() helper methods that determine the appropriate fallback based on analysis.IsRequired and analysis.Nullability.IsNullableType
• Render methods: Updated to accept a fallback string instead of a boolean, providing more flexibility
• Tests: Added 5 new tests for nullable nested collections and objects (both mapper-based and inline)

Before (caused CS8601 warnings)

Items = item.TryGetValue("items", ...) ? ... : null,  // Warning on non-nullable List<T>

After (no warnings)

// For non-nullable, non-required collections - use empty collection
Items = item.TryGetValue("items", ...) ? ... : [],

// For nullable collections - use null
Items = item.TryGetValue("items", ...) ? ... : null,

// For required properties - throw
ShippingAddress = item.TryGetValue("shippingAddress", ...) ? ... 
    : throw new System.InvalidOperationException("Required attribute 'shippingAddress' not found."),

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✅ Checklist

• My changes build cleanly
• I've added/updated relevant tests
• I've added/updated documentation or README
• I've followed the coding style for this project
• I've tested the changes locally (if applicable)

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🧪 Related Issues or PRs

N/A

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💬 Notes for Reviewers

• The fallback logic correctly distinguishes between the C# required keyword and simple non-nullability
• Empty collection expressions ([]) are used for non-required, non-nullable collections which works in C# 12+
• For non-nullable, non-required nested objects, we still use null as fallback - this is a design limitation where the model should either use required or be nullable

🤖 Generated with Claude Code

[chatgpt-codex-connector]

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[j-d-ha]

👍 LGTM