[C# Generator] Custom code (CustomCodeView) properties missing attributes from Roslyn parser

[live1206]

Bug Description

The C# generator's custom code (CustomCodeView) Roslyn parser does not extract attributes from properties defined in custom .cs files. When PropertyHelpers.GetAllProperties() returns properties from CustomCodeView, their Attributes collection is always empty — even if the custom code declares attributes like [Obsolete], [EditorBrowsable], etc.

This prevents the generator from detecting attributes on custom code properties at generation time.

Impact

The Azure SDK mgmt generator (@azure-typespec/http-client-csharp-mgmt) added logic in PR Azure/azure-sdk-for-net#57986 to skip [Obsolete] properties during @@Legacy.flattenProperty processing. However, this check (IsObsoleteProperty) always returns false for custom code properties because attributes are not populated.

Steps to Reproduce

1. Define a TypeSpec model with a property (e.g., startTimeUtc)
2. In custom code (partial class), declare the same property with [Obsolete]
3. At generation time, access modelProvider.CustomCodeView.Properties and check property.Attributes

Expected: Attributes contains the [Obsolete] attribute
Actual: Attributes is empty (attrCount=0)

Debug Evidence

From the Azure.ResourceManager.Hci SDK generation with debug logging in FlattenPropertyVisitor:

CustomCodeView props: [StartTimeUtc(obsolete=False, attrCount=0, attrTypes=[]),
                       EndTimeUtc(obsolete=False, attrCount=0, attrTypes=[]),
                       LastUpdatedTimeUtc(obsolete=False, attrCount=0, attrTypes=[])]

The custom .cs file declares:

[Obsolete("This property is now deprecated.")]
[EditorBrowsable(EditorBrowsableState.Never)]
public DateTimeOffset? StartTimeUtc { get => throw new NotSupportedException(); ... }

Expected Fix

The Roslyn parser that builds PropertyProvider instances from custom code should also extract AttributeData from IPropertySymbol and populate the Attributes collection on the resulting PropertyProvider.