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/nvidia/config.py

@@ -0,0 +1,64 @@
+# 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.
+
+import os
+from typing import Any
+
+from pydantic import Field
+
+from llama_stack.providers.utils.inference.model_registry import RemoteInferenceProviderConfig
+from llama_stack.schema_utils import json_schema_type
+
+
+@json_schema_type
+class NVIDIAConfig(RemoteInferenceProviderConfig):
+    """
+    Configuration for the NVIDIA NIM inference endpoint.
+
+    Attributes:
+        url (str): A base url for accessing the NVIDIA NIM, e.g. http://localhost:8000
+        api_key (str): The access key for the hosted NIM endpoints
+
+    There are two ways to access NVIDIA NIMs -
+     0. Hosted: Preview APIs hosted at https://integrate.api.nvidia.com
+     1. Self-hosted: You can run NVIDIA NIMs on your own infrastructure
+
+    By default the configuration is set to use the hosted APIs. This requires
+    an API key which can be obtained from https://ngc.nvidia.com/.
+
+    By default the configuration will attempt to read the NVIDIA_API_KEY environment
+    variable to set the api_key. Please do not put your API key in code.
+
+    If you are using a self-hosted NVIDIA NIM, you can set the url to the
+    URL of your running NVIDIA NIM and do not need to set the api_key.
+    """
+
+    url: str = Field(
+        default_factory=lambda: os.getenv("NVIDIA_BASE_URL", "https://integrate.api.nvidia.com"),
+        description="A base url for accessing the NVIDIA NIM",
+    )
+    timeout: int = Field(
+        default=60,
+        description="Timeout for the HTTP requests",
+    )
+    append_api_version: bool = Field(
+        default_factory=lambda: os.getenv("NVIDIA_APPEND_API_VERSION", "True").lower() != "false",
+        description="When set to false, the API version will not be appended to the base_url. By default, it is true.",
+    )
+
+    @classmethod
+    def sample_run_config(
+        cls,
+        url: str = "${env.NVIDIA_BASE_URL:=https://integrate.api.nvidia.com}",
+        api_key: str = "${env.NVIDIA_API_KEY:=}",
+        append_api_version: bool = "${env.NVIDIA_APPEND_API_VERSION:=True}",
+        **kwargs,
+    ) -> dict[str, Any]:
+        return {
+            "url": url,
+            "api_key": api_key,
+            "append_api_version": append_api_version,
+        }