Document ClientOAuthOptions.ScopeSelector usage

ClientOAuthOptions.ScopeSelector is a public API for customizing the OAuth scopes selected after the server-provided scope information has been resolved. It was added in #1596, but I couldn't find any conceptual documentation showing when or how to use it.

The XML documentation describes the API behavior, but users browsing the SDK documentation may not discover that ScopeSelector can be used to:

• filter scopes advertised by the server;
• append client-specific scopes when needed;
• return null or an empty sequence to omit the scope parameter entirely.

A small conceptual example would make this behavior easier to discover, for example:

var oauthOptions = new ClientOAuthOptions
{
    ScopeSelector = scopes =>
        scopes?.Where(scope => scope is "mcp:tools" or "mcp:resources")
};

This would be especially useful alongside the existing OAuth guidance, where users may otherwise assume that Scopes is the only way to influence the requested scope set.

I also checked the currently open OAuth documentation work in #1801, which does not appear to cover ScopeSelector.

I'd be happy to submit a focused documentation PR for this and keep the change narrowly scoped to client-side scope selection, placing it wherever best fits the documentation structure.

[!NOTE]
This issue was prepared with AI assistance.

Start with the existing OAuth guidance and the ClientOAuthOptions.ScopeSelector XML documentation, then identify the best location for a focused conceptual section. Done means documenting filtering, appending client-specific scopes, and returning null or an empty sequence to omit the scope parameter, with a small C# example.