Python: make FoundryChatClient an async context manager

[he-yufeng]

Fixes #5428.

The `built_in_chat_clients` sample (and any other code path that does `async with client:`) wraps the chat client in an async context manager. This works for `OpenAIChatClient` and the Azure variants but fails on `FoundryChatClient` with:

```
TypeError: FoundryChatClient object does not support the asynchronous context manager protocol (missed aexit method)
```

`FoundryChatClient` owns an `AIProjectClient` internally — self-created when the caller passes `project_endpoint` + `credential`, otherwise injected — so beyond the missing protocol methods, leaving without closing the self-created `project_client` also leaks it.

Fix

Mirror the pattern already used in `FoundryChatAgent` (`_agent.py`):

• Track `_should_close_client` on `RawFoundryChatClient`, flipped on only when we construct the `AIProjectClient` ourselves.
• Add `async def close()` that awaits `project_client.close()` only when owned.
• Implement `aenter` (no-op, returns self) and `aexit` (awaits close).

`FoundryChatClient` inherits these via the existing class hierarchy, so the sample starts working without any call-site changes and without touching the public API.

Test plan

• `async with FoundryChatClient(project_endpoint=..., credential=...) as client: ...` — no longer raises, and the self-created `AIProjectClient` is closed on exit.
• `async with FoundryChatClient(project_client=existing): ...` — exits cleanly without closing the caller-owned `project_client`.
• Existing `test_foundry_chat_client.py` suite continues to pass (`ruff check` / `ruff format` green locally).

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[eavanvalkenburg]

so, I need to think about this one, because currently no chat client has aenter, so this would introduce it, I'm not opposed, but need to think about this generally. I will close this for now (I removed the async with from the sample you referred to)

[eavanvalkenburg]

this is from a different PR, please remove here