Add Agentic auth handler

[sw-joelmut]

Fixes #689

Description

This PR adds the Agentic auth handler functionality, and adds support for loading the type 'agentic' from the .env, alongside a few validations of the provided handler options.

Testing

Unable to test this functionality as it is Microsoft's first-party app, but we made sure the sample validated the options and started successfully.

[Copilot]

Pull Request Overview

This PR adds support for Agentic authentication by implementing a new Agentic authorization handler alongside the existing Azure Bot authorization handler. The changes enable loading the 'agentic' type from environment variables and include validation of handler options.

Key changes:

• Implementation of AgenticAuthorization handler with token retrieval functionality
• Addition of environment variable support for authorization handler type configuration
• Enhanced error handling with proper message prefixing

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
packages/agents-hosting/src/app/auth/handlers/agenticAuthorization.ts	Complete implementation of the Agentic authorization handler with token management and validation
packages/agents-hosting/src/app/auth/authorizationManager.ts	Added support for loading authorization handler type from environment variables with validation
packages/agents-hosting/src/app/auth/handlers/azureBotAuthorization.ts	Enhanced error handling by adding message prefixing for consistency

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