Register List<object> JSON metadata for Native AOT array tool arguments

[girishkvs]

Fixes #1846

Problem

Passing an array argument to a tool throws at runtime under Native AOT:

System.NotSupportedException: JsonTypeInfo metadata for type 'System.Collections.Generic.List`1[System.Object]'
was not provided by TypeInfoResolver of type '[ModelContextProtocol.McpJsonUtilities+JsonContext, ...]'.
   at ModelContextProtocol.Client.McpClient.ToArgumentsDictionary(IReadOnlyDictionary`2 arguments, JsonSerializerOptions options)
   at ModelContextProtocol.Client.McpClient.CallToolAsync(...)

ToArgumentsDictionary serializes each argument value through McpJsonUtilities.JsonContext. A List<object> argument has no generated metadata there, so the source-generated resolver has nothing to return and serialization fails. Trimmed/AOT apps hit this on any array-valued tool parameter.

Change

Add [JsonSerializable(typeof(List<object>))] to McpJsonUtilities.JsonContext, next to the existing IDictionary<string, object> and IReadOnlyDictionary<string, object> entries.

Extend the AOT compatibility test app with a Join(string[] items) tool and invoke it with a populated and an empty List<object>, so the scenario is covered by make test-aot.

Validation

make test-aot equivalent — dotnet publish with PublishAot=true then run the produced native binary:

Build	Result
With this change	Success!, exit 0
With the JsonSerializable line reverted	NotSupportedException as above, exit non-zero

The new test app assertions fail without the one-line McpJsonUtilities change, so the regression is covered rather than assumed.

dotnet build src/ModelContextProtocol.Core is clean across netstandard2.0, net8.0, net9.0 and net10.0 — 0 warnings, 0 errors.