Extend multiple-services design doc with autoMerge option for client hierarchy customization

[Copilot]

TCGC currently only supports auto-merging multiple services into one client. This design doc update adds support for three additional scenarios: separate clients per service, services as direct children without deep merging, and fully customized client hierarchies.

Changes to multiple-services.md

• Scenario 1: Multiple independent clients, each targeting one service
• Scenario 2: New autoMerge: false option to prevent deep merging—each service namespace becomes a direct sub-client preserving its own hierarchy
• Scenario 3: Fully customized hierarchy using nested @client with explicit operation mapping via is keyword

autoMerge Behavior Summary

autoMerge	Explicit @client	Result
true (default)	No	All services' nested items merged as root client's sub-clients
false	No	Each service namespace becomes a sub-client with its own hierarchy
false	Yes	Only explicitly defined clients and operations
true	Yes	Explicit clients + remaining content auto-merged

Example

@client({
  name: "CombineClient",
  service: [ServiceA, ServiceB],
  autoMerge: false,  // NEW OPTION
})
namespace CombineClient;

With autoMerge: false:

client.service_a.operations.op_a()  # ServiceA hierarchy preserved
client.service_b.operations.op_b()  # ServiceB hierarchy preserved

Design doc only—no code changes included.

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