Add Back Compat Attribute Processing for ModelReaderWriterContextDefinition

[jorgerangel-msft]

We should check the last contract in link when building the attributes for this type. We should ensure any previous attribute that is missing is added, and we don't have duplicates. Ensure we have unit tests as well.

[github-actions]

🎯 Issue Triage

This is a feature request to improve backward compatibility handling in the C# client emitter by ensuring ModelReaderWriterContextDefinition checks the previous contract when building attributes — adding any missing attributes from the last contract and deduplicating them.

Category: Feature Request
Area: emitter:client:csharp

🔍 Analysis

• Affected component: ModelReaderWriterContextDefinition.cs in Microsoft.TypeSpec.Generator.ClientModel
• Core ask: When generating attributes for ModelReaderWriterContextDefinition, the generator should inspect the last contract (line 47 of the linked file), carry forward any attributes present in the previous contract that are missing in the new one, and avoid emitting duplicates.
• Testing: Unit tests are explicitly requested to cover both the "missing attribute added back" and "no duplicate" scenarios.
• No reproduction code applicable — this is an enhancement to the code generation logic itself.

📝 Suggested next steps

• Read the existing ModelReaderWriterContextDefinition.cs attribute-building logic (around line 47).
• Implement logic to diff current vs. last-contract attributes and merge missing ones.
• Deduplicate the resulting attribute list.
• Add unit tests covering: attribute carried forward, no duplicates, no regression when no previous contract exists.

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