Create documentation for backward compatibility supported scenarios

[Copilot]

• Analyze all usages of LastContractView in the generator codebase
• Identify all back compatibility scenarios supported by the generator
• Create comprehensive documentation in packages/http-client-csharp/generator/docs/backward-compatibility.md
• Include examples for each scenario
• Validate documentation accuracy against code and tests
• Address PR feedback and simplify documentation
• Fix markdown formatting with prettier
• Remove unsupported scenario based on team feedback
• Update sub-headers to use "Scenario:" prefix without numbers

Summary

Created documentation for backward compatibility support in the TypeSpec C# generator. The document covers:

Supported Back-Compat Scenarios:

1. Model Factory Methods

   • New model properties added - generates compatibility overload without new parameter
   • Parameter ordering changed - preserves previous parameter order

2. Model Properties

   • Collection property type changes (ReadOnly ↔ ReadWrite) - attempts to preserve previous type

3. API Version Enum

   • Removed/changed API versions - preserves previous enum members and re-indexes values

Recent Changes:

• Updated sub-headers to use "Scenario:" prefix without numbers (e.g., "Scenario 1: New Model Property Added" → "Scenario: New Model Property Added")
• Fixed markdown formatting by running prettier (adds proper blank lines before code blocks)
• Removed "Model Factory Method Renamed" scenario as it's not actually supported by the generator

The documentation is located at /packages/http-client-csharp/generator/docs/backward-compatibility.md.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Create Document on All Back Compatibility Supported Scenarios</issue_title>
<issue_description>We should create a document that lists all of the supported back compat scenarios. Scan the generator repo to determine where the LastContractView is consumed, and what actions is the generator performing with. We should come up with a simple doc that lists the high level scenarios that it supports. In example, adding back parameters in model factory methods from previous releases.

This doc should live in https://github.com/microsoft/typespec/tree/main/packages/http-client-csharp/generator/docs</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #8808

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