Add async streaming and async embeddings tests

[MitulShah1]

The async client has good coverage now (TestAsyncResources, TestAsyncCompletionsParams, async retry/context-manager tests), but two async paths still have no tests:

1. AsyncCompletions.create(stream=True) — async streaming (sync streaming is covered by TestStreamingErrorHandling; async is not).
2. AsyncEmbeddings.create() — no async embeddings test (TestEmbeddings is sync only).

What to do

Add pytest-asyncio + pytest-httpx tests for both, mirroring their sync counterparts (TestChatCompletions.test_streaming_returns_iterator and TestEmbeddings.test_create).