Fix crash when MCP plugins are persisted with Guid.Empty as Id

[CowboyHat-exe]

Problem

KernelPluginCollection uses the plugin Name (derived from Id.ToString("N")) as its dictionary key. When two or more McpChatPluginEntity records in settings have Id == Guid.Empty, the second Add call throws:

System.ArgumentException: Argument_AddingDuplicateWithKey, 00000000000000000000000000000000
   at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
   at Microsoft.SemanticKernel.KernelPluginCollection.Add(KernelPlugin plugin)
   at Everywhere.Chat.ChatService.BuildKernelAsync(...)

Every chat request fails with "An unknown error occurred" until the app is restarted with healed settings.

Root cause

McpChatPluginEntity.ToMcpChatPlugin() passes Id directly to the McpChatPlugin constructor without checking for Guid.Empty. If settings were ever written with un-initialised IDs (e.g. by a migration or external edit), all such entries produce the same plugin name and the second one crashes the kernel builder.

Fix

Self-heal on load: if Id == Guid.Empty, assign Guid.CreateVersion7() before constructing the plugin. The healed value is written back to settings on the next save (via the existing SourceList → McpChatPluginEntity binding in ChatPluginManager), so the repair is permanent.

public McpChatPlugin? ToMcpChatPlugin()
{
    var transportConfiguration = Stdio as McpTransportConfiguration ?? Http;
    if (transportConfiguration == null) return null;

    // Self-heal persisted Guid.Empty IDs; they cause KernelPluginCollection to throw
    // a duplicate-key ArgumentException when two or more zero-ID plugins are loaded.
    if (Id == Guid.Empty)
        Id = Guid.CreateVersion7();

    return new McpChatPlugin(Id, transportConfiguration);
}

Not changed: the [JsonConstructor] path — initialising Id there would assign a fresh GUID to every record on every deserialization, breaking round-trips for records that already have a valid Id.

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[DearVa]

Hey @CowboyHat-exe, thank you very much for this PR!

The root cause, however, actually stems from PluginSettings.McpChatPlugins using an inappropriate data type: it should be an ObservableDictionary<string, McpChatPluginEntity> rather than a list. That would eliminate duplicate keys at the source, while also allowing us to drop the Id field from McpChatPluginEntity entirely. Additionally, ChatPluginSnapshot itself needs name deduplication to prevent this exception from surfacing further downstream.

The refactoring involved is fairly extensive — it includes converting both ChatPluginManager._builtInPluginsSource and ChatPluginManager._mcpPluginsSource to SourceCache with custom mapping strategies. I'm planning to tackle these refactorings in the near future, along with removing the ObjectObserver-based change tracking.