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.