chore(package): migrate to src/ layout (#3920)

Migrates package structure to src/ layout following Python packaging
best practices.

All code moved from `llama_stack/` to `src/llama_stack/`. Public API
unchanged - imports remain `import llama_stack.*`.

Updated build configs, pre-commit hooks, scripts, and GitHub workflows
accordingly. All hooks pass, package builds cleanly.

**Developer note**: Reinstall after pulling: `pip install -e .`

src/llama_stack/providers/remote/inference/vllm/vllm.py

@@ -0,0 +1,111 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+from collections.abc import AsyncIterator
+from urllib.parse import urljoin
+
+import httpx
+from openai.types.chat.chat_completion_chunk import (
+    ChatCompletionChunk as OpenAIChatCompletionChunk,
+)
+from pydantic import ConfigDict
+
+from llama_stack.apis.inference import (
+    OpenAIChatCompletion,
+    OpenAIChatCompletionRequestWithExtraBody,
+    ToolChoice,
+)
+from llama_stack.log import get_logger
+from llama_stack.providers.datatypes import (
+    HealthResponse,
+    HealthStatus,
+)
+from llama_stack.providers.utils.inference.openai_mixin import OpenAIMixin
+
+from .config import VLLMInferenceAdapterConfig
+
+log = get_logger(name=__name__, category="inference::vllm")
+
+
+class VLLMInferenceAdapter(OpenAIMixin):
+    config: VLLMInferenceAdapterConfig
+
+    model_config = ConfigDict(arbitrary_types_allowed=True)
+
+    provider_data_api_key_field: str = "vllm_api_token"
+
+    def get_api_key(self) -> str | None:
+        if self.config.auth_credential:
+            return self.config.auth_credential.get_secret_value()
+        return "NO KEY REQUIRED"
+
+    def get_base_url(self) -> str:
+        """Get the base URL from config."""
+        if not self.config.url:
+            raise ValueError("No base URL configured")
+        return self.config.url
+
+    async def initialize(self) -> None:
+        if not self.config.url:
+            raise ValueError(
+                "You must provide a URL in run.yaml (or via the VLLM_URL environment variable) to use vLLM."
+            )
+
+    async def health(self) -> HealthResponse:
+        """
+        Performs a health check by verifying connectivity to the remote vLLM server.
+        This method is used by the Provider API to verify
+        that the service is running correctly.
+        Uses the unauthenticated /health endpoint.
+        Returns:
+
+            HealthResponse: A dictionary containing the health status.
+        """
+        try:
+            base_url = self.get_base_url()
+            health_url = urljoin(base_url, "health")
+
+            async with httpx.AsyncClient() as client:
+                response = await client.get(health_url)
+                response.raise_for_status()
+                return HealthResponse(status=HealthStatus.OK)
+        except Exception as e:
+            return HealthResponse(status=HealthStatus.ERROR, message=f"Health check failed: {str(e)}")
+
+    def get_extra_client_params(self):
+        return {"http_client": httpx.AsyncClient(verify=self.config.tls_verify)}
+
+    async def check_model_availability(self, model: str) -> bool:
+        """
+        Skip the check when running without authentication.
+        """
+        if not self.config.auth_credential:
+            model_ids = []
+            async for m in self.client.models.list():
+                if m.id == model:  # Found exact match
+                    return True
+                model_ids.append(m.id)
+            raise ValueError(f"Model '{model}' not found. Available models: {model_ids}")
+        log.warning(f"Not checking model availability for {model} as API token may trigger OAuth workflow")
+        return True
+
+    async def openai_chat_completion(
+        self,
+        params: OpenAIChatCompletionRequestWithExtraBody,
+    ) -> OpenAIChatCompletion | AsyncIterator[OpenAIChatCompletionChunk]:
+        params = params.model_copy()
+
+        # Apply vLLM-specific defaults
+        if params.max_tokens is None and self.config.max_tokens:
+            params.max_tokens = self.config.max_tokens
+
+        # This is to be consistent with OpenAI API and support vLLM <= v0.6.3
+        # References:
+        #   * https://platform.openai.com/docs/api-reference/chat/create#chat-create-tool_choice
+        #   * https://github.com/vllm-project/vllm/pull/10000
+        if not params.tools and params.tool_choice is not None:
+            params.tool_choice = ToolChoice.none.value
+
+        return await super().openai_chat_completion(params)