.Net: Fix #13516 - Parse plugin name from fully qualified function name in MEAI conversion

[Whning0513]

Bug: The function call plugin name printed is empty

When using Ollama with Semantic Kernel and invoking FunctionChoiceBehavior.Auto, the functionCall.PluginName is empty/blank in FunctionCallContent objects. This prevents proper identification of which plugin a function belongs to. Users must rely on the function name alone for routing, which is ambiguous when multiple plugins have functions with the same name.

Reported by: Community user

The ChatMessageExtensions.ToChatMessageContent() method in the MEAI (Microsoft.Extensions.AI) conversion layer was converting FunctionCallContent from MEAI format to Semantic Kernel format without parsing the plugin name from the fully qualified function name. The MEAI convention uses an underscore-separated format (plugin_function) but the conversion code was passing null as the plugin name.

File: dotnet/src/InternalUtilities/meai/Extensions/ChatMessageExtensions.cs

• Added GetFunctionName() static method that parses the plugin name from the underscore-separated MEAI function name format
• When an underscore is found, the portion before the first underscore is treated as the pluginName and the portion after as the functionName
• When no underscore is found, pluginName is set to null (backward compatible)
• Integrated GetFunctionName() into the FunctionCallContent constructor call to populate the pluginName parameter

Low - The change adds plugin name parsing logic that populates a field that was previously null. If the MEAI function name doesn't follow the plugin_function convention, the plugin name remains null (no behavior change). For names that do follow the convention, the correct plugin name is now available.

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