fix(streaming): add aclose() to AsyncStream for standard async cleanup protocol

[s-zx]

Problem

AsyncStream exposes close() but not aclose(), causing AttributeError when instrumentation libraries (Langfuse, OpenTelemetry wrappers, etc.) call the standard Python async cleanup convention:

await stream.aclose()
# AttributeError: 'AsyncStream' object has no attribute 'aclose'. Did you mean: 'close'?

The error surfaces in production via the call chain in AsyncChatCompletionStream:

AsyncChatCompletionStreamManager.__aexit__
  -> AsyncChatCompletionStream.close()        # line 215
  -> self._response.aclose()                  # AttributeError when _response is AsyncStream

When instrumentation wraps the raw AsyncStream, self._response resolves to the AsyncStream itself rather than the underlying httpx.Response. Since AsyncStream has close() but not aclose(), cleanup fails.

Closes #2853

Fix

Add aclose() as a thin alias that delegates to close():

async def aclose(self) -> None:
    """Async-convention alias for close()."""
    await self.close()

aclose() is the standard async cleanup method used by asyncio.StreamWriter, httpx.AsyncByteStream, and async generators (PEP 525). Adding it lets callers and wrappers use either name without special-casing AsyncStream.

The sync Stream class is not affected since its callers use close().