Support specifying OutputSchema independently of return type for tools returning CallToolResult

[Copilot]

Tools returning CallToolResult directly (for control over Meta, IsError, StructuredContent) had no way to advertise a meaningful OutputSchema in tools/list. The inferred schema would reflect CallToolResult itself rather than the actual structured content shape.

Changes

• McpServerToolCreateOptions: Add JsonElement? OutputSchema property (pre-computed schema, included in Clone())
• McpServerToolAttribute: Add Type? OutputSchemaType property (converted to JSON schema via AIJsonUtilities.CreateJsonSchema during option derivation)
• AIFunctionMcpServerTool.CreateOutputSchema: Explicit OutputSchema takes precedence over AIFunction.ReturnJsonSchema; existing wrapping/normalization logic applies to both paths
• 13 new tests covering options-based, attribute-based, CallToolResult passthrough, non-object wrapping, nullable simplification, and options-over-attribute precedence

Usage

// Attribute-based: Type is converted to JSON schema at tool creation
[McpServerTool(UseStructuredContent = true, OutputSchemaType = typeof(WeatherResponse))]
public static CallToolResult GetWeather(string city)
{
    var response = new WeatherResponse { Temperature = 72, Condition = "sunny" };
    return new CallToolResult
    {
        Content = [new TextContentBlock { Text = "72°F, sunny" }],
        StructuredContent = JsonSerializer.SerializeToElement(response),
        IsError = false,
    };
}

// Options-based: pass a pre-computed JsonElement schema
McpServerTool.Create(myDelegate, new McpServerToolCreateOptions
{
    UseStructuredContent = true,
    OutputSchema = myJsonSchemaElement,
});

UseStructuredContent = true is still required for OutputSchema/OutputSchemaType to take effect.

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[stephentoub]

@halter73 / @mikekistler, I explored creating a CallToolResult<T> and using that, but it leads to too many complications/oddities to be worthwhile in my opinion. I think the simple approach of just having OutputType on the attribute is the right answer.