Backwards compat for Copilot Studio Client

[benbrown]

In order to avoid a breaking change in the syntax of CopilotStudioClient, we introduce:

startConversationStreaming -> returns AsyncGenerator
startConversationAsync -> marked as deprecated, still returns Promise<Activity[]> as before

sendActivityStreaming -> returns AsyncGenerator
sendActivity -> marked as deprecated, still returns Promise<Activity[]>

askQuestionAsync -> marked as deprecated, returns Promise<Activity[]>

[ceciliaavila]

LGTM

[Copilot]

Pull request overview

This PR introduces streaming-based methods to maintain backwards compatibility for CopilotStudioClient while deprecating promise-based methods, aligning usage in WebChat and tests with the new APIs.

• Add startConversationStreaming and sendActivityStreaming that return AsyncGenerator.
• Keep startConversationAsync and sendActivity returning Promise<Activity[]> and mark them as deprecated, with tests updated accordingly.
• Introduce a question helper, but it currently has an API naming inconsistency.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
packages/agents-copilotstudio-client/test/copilotStudioClient.test.ts	Adds tests for deprecated promise-based methods and new streaming methods to validate behavior and headers.
packages/agents-copilotstudio-client/src/copilotStudioWebChat.ts	Updates WebChat to use streaming APIs via for-await for conversation start and activity sending.
packages/agents-copilotstudio-client/src/copilotStudioClient.ts	Implements streaming methods, wraps deprecated methods to return promises, but introduces a naming inconsistency for askQuestionAsync.

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