[http-client-csharp] Support composes client option for extension properties

[JoshLove-msft]

Description

Add support in the C# emitter for a @clientOptions option named "composes". It indicates that the decorated model provides extension properties for another client options model.

@alternateType(
  { identity: "OpenAI.Responses.CreateResponseOptions" },
  "csharp"
)
model OpenAICreateResponseOptions {}

@clientOptions({
  name: "composes",
  value: OpenAICreateResponseOptions,
  scope: "csharp"
})
model FoundryCreateResponseOptions {
  agent: AgentReference;
  agentConversationId: string;
  sessionId: string;
}

Model-valued clientOptions support is tracked separately in Azure/typespec-azure#5256.

Expected behavior

The C# emitter should generate the properties declared by FoundryCreateResponseOptions as extension properties on the alternate type associated with OpenAICreateResponseOptions, OpenAI.Responses.CreateResponseOptions. Conceptually, the output should follow this shape:

public static class FoundryCreateResponseOptionsExtensions
{
    extension (OpenAI.Responses.CreateResponseOptions _)
    {
        public AgentReference Agent { get; set; }
        public string AgentConversationId { get; set; }
        public string SessionId { get; set; }
    }
}

[github-actions]

🎯 Issue Triage

Feature request to add a new @clientOptions decorator that allows TypeSpec models to declare emitter-specific options (initially for C# extension properties via a "composes" option).

Category: Feature Request
Area: emitter:client:csharp

🔍 Analysis

• The proposed @clientOptions(name, value, scope) decorator would enable models to annotate emitter-specific behavior, similar to how @alternateType works.
• The primary motivating use case is generating C# extension properties: when a model composes an external type (via @alternateType), @clientOptions("composes", ...) would tell the C# emitter to emit extension properties on that external type rather than a standalone class.
• This requires changes to both the TypeSpec compiler/TCGC decorator layer (new @clientOptions decorator definition) and the http-client-csharp emitter (handling of the "composes" option to generate C# extension members).
• The scope parameter follows the existing pattern of @alternateType for emitter-scoping.

📝 Suggested next steps

• Design the @clientOptions decorator signature and add it to @typespec/client-core or the appropriate shared library
• Update TCGC to propagate the option metadata
• Implement "composes" handling in the C# emitter to emit extension property classes
• Add Spector test coverage for this scenario

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