Simplify setting http response after activity processing

[singhk97]

Ensure that HTTP responses are returned directly from handlers, rather than through event-based callbacks.

Activity processing and response flow:

• The onActivity handler in app.events.ts now returns a Promise<InvokeResponse> and directly returns the result of processing the activity, rather than just awaiting it.
• The $process function in app.process.ts is refactored to always return an InvokeResponse, handling both normal and error cases, and ensuring response construction is consistent.

HttpPlugin interface and implementation updates:

• The $onActivity event handler in HttpPlugin is now asynchronous and returns a Promise<InvokeResponse>, aligning with the new app contract.
• The HTTP request handler in HttpPlugin now awaits the activity response and sends it directly to the client, removing the previous pattern of storing pending responses and using callback events.
• Old error and activity response event handlers (onError, onActivityResponse) and related state management are removed from HttpPlugin, as response handling is now synchronous with the HTTP request.

Type and import consistency:

• The InvokeResponse type is imported and used consistently across files to improve type safety and clarity.

[heyitsaamir]

Makes sense to me. Add tests?