Add regression tests for extensible string enum collection serialization

[Copilot]

Issue reported that extensible string enums in collections generated incorrect serialization code (ToSerialString() / ToEnumName() extension methods) instead of the correct pattern (ToString() / constructor).

Investigation Result: The bug does not reproduce in the current codebase. The serialization logic correctly handles extensible enums:

• MrwSerializationTypeDefinition.SerializeJsonValueCore() uses ToString() for string-based extensible enums
• CSharpTypeSnippets.ToEnum() uses constructor for extensible enums (structs)

Changes

• Added CreateModelWithExtensibleStringEnumCollection() helper to create test fixtures
• Added TestArrayOfExtensibleStringEnum_UsesCorrectSerialization - validates item.ToString() pattern
• Added TestArrayOfExtensibleStringEnum_UsesCorrectDeserialization - validates new EnumType(...) pattern

Expected generated code (which is what the current implementation produces):

// Serialization - extensible enum uses ToString()
foreach (var item in EnumCollection)
{
    writer.WriteStringValue(item.ToString());
}

// Deserialization - extensible enum uses constructor
array.Add(new TestEnum(item.GetString()));

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Bug]: Missing Extension Methods for Extensible Enums in Collections</issue_title>
<issue_description>### Describe the bug

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Bug 1: Missing Extension Methods for Extensible Enums in Collections

Description

The TypeSpec C# generator produces serialization code that calls ToSerialString() and To<EnumName>() extension methods for extensible enum types (readonly structs with string backing), but these extension methods are never generated.

Reproduction

When generating code for CjkBigramTokenFilterScripts (an extensible enum defined as a readonly partial struct), the generated serialization file CjkBigramTokenFilter.Serialization.cs contains:

// Line 46 - serialization
writer.WriteStringValue(item.ToSerialString());

// Line 107 - deserialization  
array.Add(item.GetString().ToCjkBigramTokenFilterScripts());

However, the generated CjkBigramTokenFilterScripts.cs file only contains the struct definition with a ToString() method no ToSerialString() or ToCjkBigramTokenFilterScripts() extension methods are generated.

Expected Behavior

For extensible enums (readonly structs), the generator should either:

1. Generate the extension methods in a separate *.Serialization.cs file (like it does for fixed enums), OR
2. Use ToString() and the constructor directly in the serialization code:

   writer.WriteStringValue(item.ToString());
   array.Add(new CjkBigramTokenFilterScripts(item.GetString()));

Workaround

Manually create the missing extension methods:

internal static class CjkBigramTokenFilterScriptsExtensions
{
    public static string ToSerialString(this CjkBigramTokenFilterScripts value) => value.ToString();
    public static CjkBigramTokenFilterScripts ToCjkBigramTokenFilterScripts(this string value) => new(value);
}

Affected Types

This likely affects all extensible enums used in collection properties. Known affected types:

• CjkBigramTokenFilterScripts
• EdgeNGramTokenizer

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Reproduction

steps: Regenerate SDK on Azure.Search.Documents.

Example PREVIOUS autorest file: https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/search/Azure.Search.Documents/src/Generated/Models/CjkBigramTokenFilterScripts.Serialization.cs

Repo: https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/search/Azure.Search.Documents/src

Checklist

• Follow our Code of Conduct
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.
• The provided reproduction is a minimal reproducible example of the bug.</issue_description>

<agent_instructions>### Root Cause

The bug is in the C# generator (http-client-csharp). For string-based extensible enums, the generator correctly uses ToString() for direct property serialization, but when the enum is inside a collection (IList<T>, arrays), it incorrectly generates calls to ToSerialString() and To<EnumName>() extension methods that are never generated.

Key Files

1. packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Snippets/CSharpTypeSnippets.cs

   • ToSerial() and ToEnum() methods handle single values correctly

2. packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/

   • Find where collection/array serialization code is generated
   • Look for code paths that don't use CSharpTypeSnippets.ToSerial() for collection items

Expected Fix

For string-based extensible enums in collections:

• Serialization: Use item.ToString() instead of item.ToSerialString()
• Deserialization: Use new EnumName(value) instead of value.To<EnumName>()

Validation

• Generated code for IList<StringExtensibleEnum> must compile
• Existing tests must pass
• Reference: TestData/SerializationCustomizationTests/CanChangeListOfEnumPropToListOfExtensibleEnum.cs shows the correct pattern</agent_instructions>

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

@efrainretana Example repro: https://github.com/Azure/azure-sdk-for-net/blob/3df80e71cc22102f60910e9d188ca864ea18849d/sdk/search/Azure.Search.Documents/src/Generated/Models/CjkBigramTokenFilter.Serialization.cs#L111

• Fixes [Bug]: Missing Extension Methods for Extensible Enums in Collections #9435

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