azure-ai-agentserver: support hosted agents on the application-scoped endpoint

## SDK Version
`azure-ai-agentserver==1.0.0b17`

## Problem

The application-scoped Responses API endpoint (`/applications/{agent}/protocols/openai/responses`) currently returns HTTP 400 for hosted agents:

> `Application-scoped routes only support prompt agents. Agent kind 'hosted' is not supported.`

This forces hosted agents to use the project-scoped endpoint (`/api/projects/{project}/openai/responses`), which pre-persists the current user message to the conversation store **before** invoking the agent. When `_fetch_historical_items` runs on a cold start (no `MemorySaver` checkpoint), it reads back the pre-persisted message and merges it with the current input — producing a duplicate. The SDK then writes the duplicate back to the store, corrupting conversation history.

## Expected Behaviour

The application-scoped endpoint should support hosted agents. This would bypass the project routing layer entirely, eliminating the pre-persistence behaviour and the resulting duplication.