azure-ai-agentserver: Support background mode and resumable streaming for hosted agents

## Feature Request

Support `background: true` and resumable streaming (`GET /responses/{id}?stream=true&starting_after=N`) for hosted agents, matching the behavior already available for direct model invocations via the Foundry Responses API.

## Current Behavior

### Direct model invocation (works)

When calling the Responses API directly with a model deployment:

```json
POST /openai/responses
{
"model": "gpt-4o",
"stream": true,
"store": true,
"background": true,
"input": [{"role": "user", "content": "Hello"}]
}
```

- Returns immediately with `status: "in_progress"` and `"background": true`
- Every SSE event includes a `sequence_number`
- `GET /responses/{id}?stream=true&starting_after=0` replays all stored events, then continues with live events if still in progress
- Response lifecycle completes correctly (`status: "completed"`, `output` populated)

### Hosted agent invocation (does not work)

When calling the same Responses API with an `agent_reference` pointing to a hosted agent:

```json
POST /openai/responses
{
"model": "gpt-5",
"stream": true,
"store": true,
"background": true,
"agent_reference": {"type": "agent_reference", "name": "my-agent"},
"input": [{"role": "user", "content": "Hello"}]
}
```

- Returns immediately with `status: "in_progress"` but the stored response has `"background": false` — the flag is silently dropped
- `GET /responses/{id}?stream=true&starting_after=0` returns `"Streaming is not enabled for this response"`
- The response object remains stuck at `status: "in_progress"` with empty `output`, even after the agent completes and conversation items are saved
- The `azure-ai-agentserver-core` SDK (v1.0.0b16) has no reference to `background` anywhere in its source

## Why This Matters

The primary use case is **resumable/rejoinable streaming for chat UIs**. When a user:
- Refreshes the page mid-generation
- Loses network connectivity temporarily
- Opens a conversation that is still being generated in another tab

They should be able to call `GET /responses/{id}?stream=true&starting_after=0` to replay past events and continue receiving live events. This works today for direct model calls but not for hosted agents, despite both using the same `/openai/responses` API surface.

## Observations

- The SDK already assigns `sequence_number` to every `ResponseStreamEvent` via `StreamEventState` — the primitive for `starting_after` resumption is already in place
- The SDK already supports `store=true` which saves completed items to the Conversations API after stream completion
- The gap appears to be in Foundry's proxy layer between the Responses API endpoint and the hosted agent container — it doesn't buffer/store SSE events as they pass through for hosted agents the way it does for direct model calls

## Expected Behavior

`background: true` + `stream: true` should work identically for hosted agents as it does for direct model calls:
1. Foundry buffers SSE events as they pass through from the hosted agent container
2. `GET /responses/{id}?stream=true&starting_after=N` replays stored events and continues with live events
3. The response lifecycle completes correctly when the agent finishes

## Environment

- `azure-ai-agentserver-core==1.0.0b16`
- `azure-ai-agentserver-langgraph==1.0.0b16`
- API version: `2025-11-15-preview`