refactor(passthrough): use AsyncOpenAI instead of AsyncLlamaStackClient (#4085)

We'd like to remove the dependence of `llama-stack` on
`llama-stack-client`. This is a necessary step.

A few small cleanups
- Enables `embeddings` now also
- Remove ModelRegistryHelper dependency (unused)
- Consolidate to auth_credential field via RemoteInferenceProviderConfig
- Implement list_models() to fetch from downstream /v1/models

## Test Plan

Tested using this script
https://gist.github.com/ashwinb/6356463d10f989c0682ab3bff8589581

Output:
```
Listing models from downstream server...
Available models: ['passthrough/ollama/nomic-embed-text:latest', 'passthrough/ollama/all-minilm:l6-v2', 'passthrough/ollama/llama3.2-vision:11b', 'passthrough/ollama/llama3.2-vision:latest', 'passthrough/ollama/llama-guard3:1b', 'passthrough/o
llama/llama3.2:1b', 'passthrough/ollama/all-minilm:latest', 'passthrough/ollama/llama3.2:3b', 'passthrough/ollama/llama3.2:3b-instruct-fp16', 'passthrough/bedrock/meta.llama3-1-8b-instruct-v1:0', 'passthrough/bedrock/meta.llama3-1-70b-instruct
-v1:0', 'passthrough/bedrock/meta.llama3-1-405b-instruct-v1:0', 'passthrough/sentence-transformers/nomic-ai/nomic-embed-text-v1.5']

Using LLM model: passthrough/ollama/llama3.2-vision:11b

Making inference request...

Response: 4.

--- Testing streaming ---
Streamed response: ChatCompletionChunk(id='chatcmpl-64', choices=[Choice(delta=ChoiceDelta(content='1', reasoning_content=None, refusal=None, role='assistant', tool_calls=None), finish_reason='', index=0, logprobs=None)], created=1762381674, m
odel='passthrough/ollama/llama3.2-vision:11b', object='chat.completion.chunk', usage=None)
...
5ChatCompletionChunk(id='chatcmpl-64', choices=[Choice(delta=ChoiceDelta(content='', reasoning_content=None, refusal=None, role='assistant', tool_calls=None), finish_reason='stop', index=0, logprobs=None)], created=1762381674, model='passthrou
gh/ollama/llama3.2-vision:11b', object='chat.completion.chunk', usage=None)
```

src/llama_stack/providers/remote/inference/passthrough/config.py

@@ -6,7 +6,7 @@
 
 from typing import Any
 
-from pydantic import Field, SecretStr
+from pydantic import Field
 
 from llama_stack.providers.utils.inference.model_registry import RemoteInferenceProviderConfig
 from llama_stack.schema_utils import json_schema_type
@@ -19,11 +19,6 @@ class PassthroughImplConfig(RemoteInferenceProviderConfig):
         description="The URL for the passthrough endpoint",
     )
 
-    api_key: SecretStr | None = Field(
-        default=None,
-        description="API Key for the passthrouth endpoint",
-    )
-
     @classmethod
     def sample_run_config(
         cls, url: str = "${env.PASSTHROUGH_URL}", api_key: str = "${env.PASSTHROUGH_API_KEY}", **kwargs