Handle collection-backed C# error messages

[AayushP123]

Summary

Fixes #7926.

The primary error-message path builder treated collection properties like single model properties. For errors such as List<ProcessResult> ProcessResults, that produced ProcessResults?.MessageEscaped, which does not compile.

Changes

• Allow language writers to customize a primary-message path segment based on whether the traversed type is a collection.
• Use FirstOrDefault() when the C# error-message path crosses a collection.
• Add System.Linq only to affected C# error models.
• Cover both the generated property expression and conditional import refinement.

Testing

• dotnet test tests/Kiota.Builder.Tests/Kiota.Builder.Tests.csproj --filter "FullyQualifiedName~Writers.CSharp.CodePropertyWriterTests|FullyQualifiedName~Refiners.CSharpLanguageRefinerTests|FullyQualifiedName~Writers.ProprietableBlockExtensions" --no-restore
• dotnet test tests/Kiota.Builder.Tests/Kiota.Builder.Tests.csproj --filter FullyQualifiedName!~KiotaSearcherTests --no-restore (2,185 passed, 2 skipped)
• dotnet format kiota.slnx --include src/Kiota.Builder/Refiners/CSharpRefiner.cs src/Kiota.Builder/Writers/CSharp/CodePropertyWriter.cs src/Kiota.Builder/Writers/ProprietableBlockExtensions.cs tests/Kiota.Builder.Tests/Refiners/CSharpLanguageRefinerTests.cs tests/Kiota.Builder.Tests/Writers/CSharp/CodePropertyWriterTests.cs --no-restore --verify-no-changes
• Generated the full attached Dayforce client and built it against Kiota abstractions and serializers with zero warnings and zero errors.

[Copilot]

Pull request overview

This PR fixes C# code generation for ApiException.Message overrides when the primary error message is reached through a collection property (e.g., List<T>), which previously produced invalid member-access code that did not compile.

Changes:

• Extends the primary-error-message path builder to allow language-specific segment customization based on whether the traversed type is a collection.
• Updates the C# writer to insert FirstOrDefault() when the primary-message path crosses a collection, producing compilable null-safe access.
• Updates the C# refiner to add System.Linq only for affected error models and adds/updates unit tests covering both the generated expression and the conditional import.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/Kiota.Builder.Tests/Writers/CSharp/CodePropertyWriterTests.cs	Adds coverage for Message override generation when the primary message is nested behind a collection property.
tests/Kiota.Builder.Tests/Refiners/CSharpLanguageRefinerTests.cs	Adds coverage ensuring System.Linq is imported only when FirstOrDefault() is required by the generated primary-message path.
src/Kiota.Builder/Writers/ProprietableBlockExtensions.cs	Adds an optional pathSegmentFactory to customize path segments per traversed type (e.g., collection vs non-collection).
src/Kiota.Builder/Writers/CSharp/CodePropertyWriter.cs	Uses pathSegmentFactory to emit ?.FirstOrDefault()?. when the primary-message path crosses a collection in C#.
src/Kiota.Builder/Refiners/CSharpRefiner.cs	Conditionally adds System.Linq for error models whose primary-message path requires FirstOrDefault().
CHANGELOG.md	Documents the C# behavior change for primary error messages nested in collections.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated no new comments.