Python: [Samples][Python] durabletask/02_multi_agent requirements.txt references wrong package (foundry instead of openai)

## Description

`python/samples/04-hosting/durabletask/02_multi_agent/` uses `OpenAIChatCompletionClient` from `agent_framework.openai`, but its `requirements.txt` references `packages/foundry` instead of `packages/openai`.

This currently works by accident because `agent-framework-openai` is a transitive dependency of `agent-framework-core`, but it's misleading for developers reading the requirements file and would break if the transitive dependency changes.

## Mismatch Details

**Code (`worker.py`):**
```python
from agent_framework.openai import OpenAIChatCompletionClient
```

**`requirements.txt` (current — incorrect):**
```
# agent-framework-foundry ← wrong commented PyPI name
-e ../../../../packages/foundry ← wrong local package
```

**`requirements.txt` (should be):**
```
# agent-framework-openai ← correct commented PyPI name
-e ../../../../packages/openai ← correct local package
```

## Suggested Fix

Update `python/samples/04-hosting/durabletask/02_multi_agent/requirements.txt`:
- Change `-e ../../../../packages/foundry` to `-e ../../../../packages/openai`
- Change the commented PyPI line from `# agent-framework-foundry` to `# agent-framework-openai`

## Environment

- Python 3.13.13, Windows 11