.Net: [.NET] Fix missing plugin name when converting function calls

[OiPunk]

What

Fixes function-call conversion from Microsoft.Extensions.AI.ChatMessage so plugin/function names are preserved instead of collapsing into a single FunctionName string.

Why

ChatMessageExtensions.ToChatMessageContent(...) previously mapped FunctionCallContent.Name directly into FunctionCallContent.FunctionName and always left PluginName empty.

This breaks scenarios where providers return fully-qualified tool names (for example time_ReadFile), producing empty plugin names in user code and making invocation/diagnostics inconsistent.

Changes

• Parse function names into (plugin, function) during conversion.
• Support separators in this order:
  • .
  • -
  • _ (safe fallback only when there is a single underscore and the function segment is PascalCase).
• Apply the same parsing when mapping FunctionResultContent from the matched function call.
• Add regression tests in ChatMessageExtensionsTests for:
  • underscore-qualified names (issue scenario)
  • snake_case names that should remain unsplit
  • function result propagation of parsed plugin/function names

Verification

• /Users/liweiguang/.dotnet/dotnet test dotnet/src/SemanticKernel.UnitTests/SemanticKernel.UnitTests.csproj --filter ChatMessageExtensionsTests --nologo
• /Users/liweiguang/.dotnet/dotnet test dotnet/src/SemanticKernel.UnitTests/SemanticKernel.UnitTests.csproj --filter ChatMessageExtensionsTests --collect:"XPlat Code Coverage" --results-directory /tmp/sk-13516-coverage --nologo

Closes #13516