McpMetaAttribute documentation was unclear about which MCP operations include the specified metadata. Per discussions in #1228 and #1230, clarify that metadata appears only in listing operations, not invocation results.

Changes

• Updated XML docs to explicitly state metadata populates Tool.Meta, Prompt.Meta, Resource.Meta for listing operations (tools/list, prompts/list, resources/list)
• Added clarification that metadata is not propagated to invocation results (tools/call, prompts/get, resources/read)
• Documented how to include metadata in invocation results (set Meta directly on result types)

Example

To include metadata in a tool call result, return CallToolResult with Meta set directly:

[McpServerTool]
[McpMeta("version", "1.0")]  // Only appears in tools/list
public CallToolResult MyTool(string input)
{
    return new CallToolResult
    {
        Content = [new TextContentBlock { Text = "result" }],
        Meta = new JsonObject { ["version"] = "1.0" }  // Appears in tools/call
    };
}

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