Add copy constructors and Clone methods to the various .NET config option bags

[stephentoub]

Consuming libraries like Agent Framework sometimes have a need to clone the various options bags, e.g. their caller passes in options and that middle library needs to tweak the settings before passing them along (e.g. set Streaming to true or false) but it doesn't want to mutate the caller's object. Without clone methods, such libraries need to manually copy every property, which then means when new properties are added, they get ignored and options are lost.

This PR adds such public Clone methods, and accomodates the non-sealed nature of the types by adding protected copy constructors that the virtual Clone methods use. (If instead we want to seal these types, that'd be viable as well.)

[Copilot]

Pull request overview

This PR adds cloning support to key .NET SDK option/config “bags” to let middle-layer libraries adjust settings without mutating caller-provided instances, by introducing protected copy constructors and public virtual Clone() methods.

Changes:

• Added protected copy constructors for CopilotClientOptions, SessionConfig, ResumeSessionConfig, and MessageOptions.
• Added public virtual Clone() methods that use those copy constructors.
• Implemented defensive copies for several collection properties (arrays/lists/dictionaries).

Comments suppressed due to low confidence (1)

dotnet/src/Types.cs:878

• Same as in SessionConfig: cloning McpServers with new Dictionary<string, object>(...) drops any custom key comparer from the original dictionary. If callers can pass in dictionaries with non-default comparers, preserve that comparer when cloning from a Dictionary<string, object> instance.

        InfiniteSessions = other.InfiniteSessions;
        McpServers = other.McpServers is not null ? new Dictionary<string, object>(other.McpServers) : null;
        Model = other.Model;