.NET: accept JSON string inline skill args

[he-yufeng]

Fixes #6020.

Summary

• parse inline skill arguments when an OpenAI-compatible backend sends the function-call arguments as a JSON string containing an object
• keep the existing fast-fail behavior for non-object arguments and invalid JSON strings
• add regression coverage for the JSON-string object path and the invalid JSON string path

To verify

• dotnet restore tests\Microsoft.Agents.AI.UnitTests\Microsoft.Agents.AI.UnitTests.csproj
• dotnet test --project tests\Microsoft.Agents.AI.UnitTests\Microsoft.Agents.AI.UnitTests.csproj -- --filter-class Microsoft.Agents.AI.UnitTests.AgentSkills.AgentInlineSkillScriptTests
• dotnet build src\Microsoft.Agents.AI\Microsoft.Agents.AI.csproj --no-restore
• dotnet format agent-framework-dotnet.slnx --include src\Microsoft.Agents.AI\Skills\Programmatic\AgentInlineSkillScript.cs tests\Microsoft.Agents.AI.UnitTests\AgentSkills\AgentInlineSkillScriptTests.cs --verify-no-changes
• git diff --check

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds support for inline skill script arguments provided as a JSON-encoded string (in addition to a JSON object), by attempting to parse string-valued JsonElement arguments into JSON before validating they form an object.

Changes:

• In ConvertToFunctionArguments, when the incoming JsonElement is a String, parse it as JSON and use the resulting root element; throw InvalidOperationException on parse failure.
• Clone JsonElement property values when sourced from the locally-parsed JsonDocument so they remain valid after the document is disposed in finally.
• Add two unit tests covering the JSON-string-of-object happy path and the invalid-JSON-string error path.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
dotnet/src/Microsoft.Agents.AI/Skills/Programmatic/AgentInlineSkillScript.cs	Accept JSON-string arguments by parsing them, with cloning + disposal to manage document lifetime.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/AgentInlineSkillScriptTests.cs	Tests for parsed-string arguments and invalid-JSON-string error.