.Net: fix function enum argument deserialization

[he-yufeng]

Summary

Fixes #13589.

KernelJsonSchemaBuilder builds function parameter schemas with the default schema options, including string enum handling. When a tool call comes back as JSON and no custom JsonSerializerOptions are supplied, KernelFunctionFromMethod.TryToDeserializeValue was using plain System.Text.Json defaults instead, so nested enum values like "hours" failed to bind.

This patch reuses the schema builder's default options as the fallback for argument deserialization, while preserving caller-provided serializer options when they exist.

I also checked the previous PR for this issue (#13623). It was closed by the author due inactivity rather than rejected, so this is a fresh patch with a smaller test surface.

Validation

dotnet test dotnet\src\SemanticKernel.UnitTests\SemanticKernel.UnitTests.csproj --framework net10.0 --filter "FullyQualifiedName~KernelFunctionFromMethodTests1" --no-restore --logger "console;verbosity=minimal"
# passed: 75

dotnet format dotnet\src\SemanticKernel.UnitTests\SemanticKernel.UnitTests.csproj --include dotnet/src/InternalUtilities/src/Schema/KernelJsonSchemaBuilder.cs dotnet/src/SemanticKernel.Core/Functions/KernelFunctionFromMethod.cs dotnet/src/SemanticKernel.UnitTests/Functions/KernelFunctionFromMethodTests1.cs --verify-no-changes --no-restore

git diff --check

[github-actions]

Automated Code Review

Reviewers: 4 | Confidence: 91% | Result: All clear

Reviewed: Correctness, Security Reliability, Test Coverage, Design Approach

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Automated review by he-yufeng's agents

[Copilot]

Pull request overview

Aligns default tool-argument deserialization with the JSON options used for tool schema generation so string-enum values (including nested enums) deserialize correctly when callers don’t provide JsonSerializerOptions.

Changes:

• Reuses KernelJsonSchemaBuilder default JsonSerializerOptions as the fallback in KernelFunctionFromMethod.TryToDeserializeValue when no options are provided.
• Exposes an internal accessor for the schema builder’s default options (with AOT annotations preserved).
• Adds a unit test covering string-enum deserialization with default options.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
dotnet/src/SemanticKernel.Core/Functions/KernelFunctionFromMethod.cs	Uses an “effective” JsonSerializerOptions fallback so enums deserialize as strings by default.
dotnet/src/InternalUtilities/src/Schema/KernelJsonSchemaBuilder.cs	Adds an internal accessor for the schema builder’s cached default serializer options.
dotnet/src/SemanticKernel.UnitTests/Functions/KernelFunctionFromMethodTests1.cs	Adds a regression test validating default string-enum argument deserialization.

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[he-yufeng]

Addressed both review comments and rebased onto current main. The unused local in the new regression test now uses an explicit discard, and the shared fallback options accessor is now named GetDefaultJsonSerializerOptions() so it no longer reads as serialization-only.\n\nValidation on Windows:\n\npowershell\ndotnet build dotnet\\src\\SemanticKernel.Core\\SemanticKernel.Core.csproj --no-restore\ndotnet build dotnet\\src\\SemanticKernel.UnitTests\\SemanticKernel.UnitTests.csproj --no-restore --no-incremental\ndotnet test dotnet\\src\\SemanticKernel.UnitTests\\SemanticKernel.UnitTests.csproj --no-restore --filter FullyQualifiedName~ItCanDeserializeStringEnumsWithDefaultJsonOptionsAsync --no-build\ndotnet format dotnet\\src\\SemanticKernel.Core\\SemanticKernel.Core.csproj --verify-no-changes --no-restore --include dotnet\\src\\InternalUtilities\\src\\Schema\\KernelJsonSchemaBuilder.cs dotnet\\src\\SemanticKernel.Core\\Functions\\KernelFunctionFromMethod.cs\ndotnet format dotnet\\src\\SemanticKernel.UnitTests\\SemanticKernel.UnitTests.csproj --verify-no-changes --no-restore --include dotnet\\src\\SemanticKernel.UnitTests\\Functions\\KernelFunctionFromMethodTests1.cs\ngit diff --check\n\n\nResult: core build passed, unit-test project build passed, targeted test passed, targeted format checks passed, diff check passed. Note: solution-wide dotnet format dotnet\SK-dotnet.slnx --no-restore is not usable in this checkout without restoring unrelated projects; it fails before formatting the touched files because many unrelated projects are unloaded.