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/openai/__init__.py

@@ -0,0 +1,15 @@
+# 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 .config import OpenAIConfig
+
+
+async def get_adapter_impl(config: OpenAIConfig, _deps):
+    from .openai import OpenAIInferenceAdapter
+
+    impl = OpenAIInferenceAdapter(config=config)
+    await impl.initialize()
+    return impl

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

@@ -0,0 +1,39 @@
+# 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 typing import Any
+
+from pydantic import BaseModel, Field
+
+from llama_stack.providers.utils.inference.model_registry import RemoteInferenceProviderConfig
+from llama_stack.schema_utils import json_schema_type
+
+
+class OpenAIProviderDataValidator(BaseModel):
+    openai_api_key: str | None = Field(
+        default=None,
+        description="API key for OpenAI models",
+    )
+
+
+@json_schema_type
+class OpenAIConfig(RemoteInferenceProviderConfig):
+    base_url: str = Field(
+        default="https://api.openai.com/v1",
+        description="Base URL for OpenAI API",
+    )
+
+    @classmethod
+    def sample_run_config(
+        cls,
+        api_key: str = "${env.OPENAI_API_KEY:=}",
+        base_url: str = "${env.OPENAI_BASE_URL:=https://api.openai.com/v1}",
+        **kwargs,
+    ) -> dict[str, Any]:
+        return {
+            "api_key": api_key,
+            "base_url": base_url,
+        }

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

@@ -0,0 +1,38 @@
+# 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 llama_stack.log import get_logger
+from llama_stack.providers.utils.inference.openai_mixin import OpenAIMixin
+
+from .config import OpenAIConfig
+
+logger = get_logger(name=__name__, category="inference::openai")
+
+
+#
+# This OpenAI adapter implements Inference methods using OpenAIMixin
+#
+class OpenAIInferenceAdapter(OpenAIMixin):
+    """
+    OpenAI Inference Adapter for Llama Stack.
+    """
+
+    config: OpenAIConfig
+
+    provider_data_api_key_field: str = "openai_api_key"
+
+    embedding_model_metadata: dict[str, dict[str, int]] = {
+        "text-embedding-3-small": {"embedding_dimension": 1536, "context_length": 8192},
+        "text-embedding-3-large": {"embedding_dimension": 3072, "context_length": 8192},
+    }
+
+    def get_base_url(self) -> str:
+        """
+        Get the OpenAI API base URL.
+
+        Returns the OpenAI API base URL from the configuration.
+        """
+        return self.config.base_url