Documents how to use ClientOAuthOptions.ScopeSelector when configuring OAuth for an HTTP MCP client.

Motivation and Context

ClientOAuthOptions.ScopeSelector is a public API, but its behavior was only described in XML documentation and was not covered in the conceptual documentation.

This update makes the API easier to discover and explains the distinction between:

• Scopes, which acts as a fallback when the server does not provide scope information; and
• ScopeSelector, which can filter or customize the resolved scope set.

It also documents that the selector runs after offline_access is appended when advertised, that the selected scopes are used for Dynamic Client Registration, and that returning null or an empty sequence omits the scope parameter.

Closes #1867.

How Has This Been Tested?

• git diff --check
• Release build succeeded locally
• Tests: 9,090 total, 8,991 passed, 99 skipped, 0 failed
• DocFX completed with --warningsAsErrors true: 0 warnings, 0 errors

The repository currently reports NU1902 for the existing Microsoft.Build.Tasks.Git 8.0.0 dependency during restore. Local build and documentation validation suppressed only that existing NuGet audit warning.

Breaking Changes

None. This is a documentation-only change.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

The change is intentionally scoped to the existing Streamable HTTP client documentation to avoid overlapping with the broader OAuth documentation work in #1801.